


- ~~ = 

NNR NN Nn nasa neakaaaars 

- i seni N mr lin ae Snr tt ern ne 

ee ert age Chin nent NT NR Ne me eae 




















© Raymond Pettibon 


RESEARCH LIBRARY 
THE GETTY RESEARCH INSTITUTE 


JOHN MOORE ANDREAS COLOR CHEMISTRY LIBRARY FOUNDATION 


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INTERNATIONAL CHEMICAL SERIES 
H. P. TALBOT, Px.D., Sc.D., Consuttine Epitror 


THE 
THEORY AND APPLICATION 
OF 
COLLOIDAL BEHAVIOR 











INTERNATIONAL CHEMICAL SERIES 


(H. P. Tarsot, P#.D., Sc.D., Consuttina Ep1Tor) 


Bancroft— 
APPLIED COLLOID CHEM- 
ISTRY 


Bingh 
FLUIDITY AND PLASTICITY 


Cady— 
INORGANIC CHEMISTRY 


Cady— 
GENERAL CHEMISTRY 


Grifin— 
TECHNICAL METHODS OF 
ANALYSIS 
As Employed in the Labora- 
tories of Arthur D. Little, Inc. 


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CHEMICAL AND METALLO- 
GRAPHIC EXAMINATION 
OF IRON, STEEL AND 
BRASS 


Hamilton and Simpson— 
CALCULATIONS OF QUAN- 


TITATIVE CHEMICAL 
ANALYSIS 
Loeb— 
PROTEINS AND THE 


THEORY OF COLLOIDAL 
BEHAVIOR 
SECOND EDITION 


Lord and Demorest— 
etter PES ANALY- 
Fifth Edition 


Mahin— 
QUANTITATIVE ANALYSIS 
Third Edition 


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QUANTITATIVE AGRICUL- 
TURAL ANALYSIS 


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COLLEGES 


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HISTORY OF CHEMISTRY 


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LEGES 


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Reape be. CHEMIS- 


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EXPERIMENTAL ORGANIC 
CHEMISTRY 
Second Edition 


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ANALYSIS OF FUEL, GAS, 
WATER AND LUBRICANTS 
Third Edition 


Robinson— 
THE ELEMENTS OF FRAC- 
TIONAL DISTILLATION 


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TECHNICAL GAS AND FUEL 
ANALYSIS 
Second Edition 
Williams— 
PRINCIPLES OF METALLO- 
GRAPHY 
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FOOD ANALYSIS 
Second Edition 


Long and Anderson— 
CHEMICAL CALCULATIONS 


Bogue— 

THE THEORY AND APPLI- 
CATION OF COLLOIDAL 
BEHAVIOR 

Two Volumes 


Reedy— 

ELEMENTARY QUALITA- 
TIVE ANALYSIS FOR 
COLLEGE STUDENTS 

Leighou— 
CHEMISTRY OF ENGINEER- 
ING MATERIALS 
Second Edition 
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PRACTICE OF ORGANIC 

CHEMISTRY 
Eucken, Jette and LaMer— 

FUNDAMENTALS OF PHY- 

SICAL CHEMISTRY 





THE 
THEORY AND APPLICATION 


OF 


COLLOIDAL BEHAVIOR 


Contributed by the foremost authorities in 
each division of the subject 


ROBERT HERMAN BOGUEH, Pu.D. (Epiror) 


Director of Research for the Portland Cement Association; Formerly Associate 
Professor of Chemistry at Lafayette College 


VOLUME I 
THE THEORY OF COLLOIDAL BEHAVIOR 


First Epirion 
SECOND IMPRESSION 


~McGRAW-HILL BOOK COMPANY, Inc. 
NEW YORK: 370 SEVENTH AVENUE 
LONDON: 6 & 8 BOUVERIE ST., E. C. 4 
1924 


CopyRiGcut, 1924, By THE 
McGraw-Hiitt Book Company, Inc. 


PRINTED IN THE UNITED STATES OF AMERICA 


THE MAPLE PRESS COMPANY, YORK, PA. 





PREFACE 


As recently as ten to fifteen years ago, “colloid chemistry” 
received none but the most casual reference, either in the liter- 
ature, or in the text-books on physical chemistry. It was not 
recognized, at that time, as a subject deserving of any especial or 
involved consideration. And yet it had been fifty years since 
Thomas Graham pointed out the essential characteristics of the 
colloid state. 

In those fifty years, investigators had not altogether neglected 
this condition of matter. But, in their studies of it, they had not 
understood the fundamental laws governing the behavior of the 
colloid state, and so failed entirely to build up a rational theory. 
Their work pointed only to empirical effects which were not 
explained by the laws of classical chemistry. The real contribu- 
tions during this period were, therefore, confined to certain 
physical effects of particle size. Since no general theory was 
evolved, any intelligent application based upon colloidal behavior 
was impossible. This explains the general neglect of the subject, 
during that period, by the average physical and industrial 
chemist. 

The first physico-chemical theory that could be applied 
directly to the explanation of colloidal behavior was suggested 
by Professor F. G. Donnan of the University of London in 
1911, and three years later Professor Henry Procter of the 
University of Leeds made use of the theory to explain the swelling 
of gelatin. This was the first attempt to account stoichiomet- 
rically, and by a principle of physical chemistry which was 
based upon the undisputed laws of thermodynamics, for a 
property that is recognized as distinctly and uniquely colloidal. 
The outstanding work of the late Doctor Jacques Loeb, during 
the past six years, has carried over the explanation, also upon 
strictly stoichiometric grounds, and upon the principle laid down 
by Donnan, to other properties which may be regarded as char- 
acteristic of the colloid state, at least insofar as the behavior of 

V 


vl PREFACE 


the proteins is concerned. That this principle is not confined to 
the protein colloids, but applies also, and with equal completeness 
and surety, to the lyophobic colloids, has been postulated by John 
Arthur Wilson, and experimental verification has been produced 
by Professor H. T. Beans and his students at Columbia Univer- 
sity, working particularly with gold sols. The relation of particle 
size to the general theory has been pointed out by the editor 
of this work. This brings us to the present moment in our 
attempt to explain colloidal behavior on a rational basis. There 
must follow much intensive work to establish the validity of a 
general theory and the field must be broadened to include the 
non-aqueous systems. This should be accomplished in the next 
few years. 

Meanwhile the industrial chemist, the agricultural chemist, 
and the physiological chemist had not been idle but, sensing the 
importance of the new science, for it was now growing so rapidly 
that it could be regarded as such, they had, in a multitude of 
cases, applied these principles to their individual problems, and 
were meeting with astonishing and successful results. As these 
multiplied, the demand for more information, both of a theoret- 
ical and of a practical nature, became apparent. A few books 
appeared bearing on special phases of the matter, as those by 
Bancroft, Loeb, Wilson, and the present editor. But a compre- 
hensive treatise was demanded, and this book is an attempt to 
meet that need. 

In a subject of such universal application, it is quite impossible 
for one man to become so well informed on each aspect as to 
justify his attempt to write such a book by his own hand alone. 
The cooperative treatise presents certain inherent difficulties, 
such as an overlapping of material presented, occasional differ- 
ences in point of view, and differences of opinion as to relevant 
subject matter. While these cannot be entirely avoided, and it is 
not always desirable that they should be avoided, the advantages 
of such a cooperative undertaking are overwhelmingly obvious. 

While attention has been given to covering every important 
theoretical aspect, and many of the most conspicuous of the 
direct applications that have been made in industry, agriculture, 
geology, medicine, etc., yet, in the treatment of the applied field, 
a great many subjects that are unquestionably concerned with 


PREFACE vil 


colloid phenomena are not given space in this treatise. There are 
three reasons for this. First, to cover every possible application 
would necessitate a series of many volumes, and as the size and 
expense increase, the usefulness to the average chemist or investi- 
gator diminishes. Second, the science is still so young that 
chemists thoroughly familiar with the modern colloid point of 
view are to be found but rarely in industry. Third, a few typical 
instances, such as are contained in the chapters of Volume II, 
will serve very well to draw to the attention of chemists the 
remarkable rédle that colloidal behavior plays in nearly every 
phase of living or inanimate processes. The editor makes no 
apology for not including the many hypotheses and experimental 
findings that have been published from time to time, but which 
have been found to be in error, and, therefore, to contribute noth- 
ing of value to the modern concept. 

This plan has evolved a work that has a direct and purposeful 
objective, and it will prove the more useful to the greater number 
of students of colloidal behavior. Every large college and uni- 
versity is now offering some work, at least to graduate students, 
in colloid chemistry. Many books are available for an elemen- 
tary presentation. But no attempt has heretofore been made to 
gather together material for an advanced text or reference work, 
nor to present the actual application of the science. The editor 
believes that these ends are achieved by the publication of this 
book. It is written for the student and the investigator, the 
research man and the practical man, who, in science or in industry, 
is concerned with the problems of colloidal behavior. 

It is a pleasure to acknowledge the hearty cooperation of the 
contributors who have made this work possible. Whatever of 
value may lie in the book, the credit belongs to the men who have 
given generously of their time and energy in the furtherance of a 
cause to which they are devoting their lives. 

The editor feels deeply the untimely death of our friend 
and associate, Doctor Jacques Loeb. The masterful chapter 
which he contributed for this book constitutes the finest 
exposition of his matured concept of colloidal behavior that has 
come from his pen. It was his last writing on the subject, and 
the most clearly and concisely expressed. New workers must 
earry on where he left off. | 














my 


° 


CONTENTS 


Volume I. THE THEORY OF COLLOIDAL BEHAVIOR 


PREFACE. 


Cuap. 


III. 


. THE THEORY ( OF ena BY Jou: H. Sirians 
. EMULSIONS AND Foams, BY Harry N. Houmss. 


. Mutruau Ratio ne ic OF Sverre: BY ae te W. Ter atie P 
. Enzymes, BY E. FRANKLAND et ee ; 


HETEROGENEOUS EQUILIBRIA 


APPLICATION OF THE THEMODYNAMICS OF HETEROGENEOUS 
EQUILIBRIA TO THE THEORY OF COLLOIDAL PHENOMENA, BY 
JOHN ARTHUR WILSON . ’ 
CRYSTALLOIDAL AND Gartdin at Pres OF ie BY 
JACQUES LOEB... : er es 
THE FLOcCULATION AND Serer OF Konrpiny es SUSPEN- 
SIONS, BY JOHN H. NortTHROP . . 


. THE Beek. BEHAVIOR OF THE Bar eLeapee BY DON AED. }): 


Van SLYKE... 


SURFACE KINETICS 


. Tur Kinetics or DispmERsE Systems, BY EK. FRANKLIN BURTON 
. SuRFACE ENERGY IN COLLOID ie By WiuiuraAM D. Har- 
. 142 
yy 
, 222 


SUNY Saas 


ADSORPTION AND CATALYSIS 


. ADSORPTION IN CoLLorp Systems, By LErEONOR MICHAELIS 
. ADSORPTION AND CATALYSIS, BY WILDER D. BANCROFT. 
. CoLLoip CHEMISTRY AND Contact CaTatysis, By Huau S. 


TAYLOR. 


. SENSITIZATION BY MEANS OF Feo setin Reteor BY Vas 
. 207 
. 324 
. 362 


FREUNDLICH. 


STRUCTURE 


. JELLIES AND GELATINOUS PRECIPITATES, BY Harry B. WEISER. 
. Tue Stupy or SoAP SOLUTIONS AND ITS BEARING UPON COLLOID 


CHEMISTRY, BY JAMES W. McBarn. 


. VISCOUS AND PLASsTiIc FLow IN COLLOID eee BY Tieng C. 


BINGHAM . 
(Complete pilex: for Paaiihore cand! eaten in eek NS 
follows p. 444). 

xl 


PaaGgE 


123 


233 


. 258 


276 


317 


410 


430 





COLLOIDAL BEHAVIOR 


CHAPTER I 


APPLICATION OF THE THERMODYNAMICS OF HETERO- 
GENEOUS EQUILIBRIA TO THE THEORY OF 
COLLOIDAL PHENOMENA 


By 


JOHN ARTHUR WILSON 


Much of the complexity of colloidal phenomena may be 
ascribed to the polyphase nature of the systems with which they 
are associated, the difficulty of recognizing and locating the 
boundaries of all of the phases present, and to the presence of 
ions, or electrically charged masses, the free diffusion of which 
between the several phases is prevented by one cause or another. 
Although the reactions occurring within the boundaries of any 
single phase may be relatively very simple, yet the behavior of 
the polyphase system, as a whole, may appear bewilderingly 
complex if the existence of any one phase is overlooked. In this 
chapter we shall present the viewpoint of a growing school of 
thought that looks upon colloidal phenomena as a manifestation 
of certain types of heterogeneous equilibria determinable quanti- 
tatively by means of thermodynamics and well-established laws 
of physical chemistry. Membrane, jelly, and surface equilibria 
will be discussed in turn. 


MEMBRANE EQUILIBRIA 


In 1911, Donnan! propounded a theory, based upon the well- 
known distribution law, to account for the type of equilibrium 
resulting from the separation by a membrane of two solutions, 
one of which contains an ionogen having one ion that cannot 


1 Donnan, F. G.: Z. Elektrochem., 17 (1911), 572. 
i! 


2 COLLOIDAL BEHAVIOR 


diffuse through the membrane, which is permeable to ail other 
ions of the system. As an example, Donnan takes an aqueous 
solution of a salt NaR, such as Congo red, in contact with a 
membrane which is impermeable to the anion R’ and the non- 
ionized salt NaR, but will allow water and Na*, or any other ion, 
to pass freely through it. The membrane separates the Congo 
red solution from an aqueous solution of sodium chloride, which 
will diffuse from its Solution II into the Solution I of NaR. 
The first problem is to determine how the sodium chloride will 
distribute itself between the solutions on the two sides of the 
membrane. When equilibrium is established, if a small virtual 
change is made reversibly at constant temperature and volume, 
the free energy will remain unchanged; that is, no work will be 
done. The change here considered is the transfer of dn mols of 
Nat and Cl’ from II to I. The work, which equals zero, is 
[Nat], [(Cl'}n 
[Nat], + dn. RT. log. [Cl’}, =) 
whence [Nat]; X.[Cl, = [Nath x Gre 


dn. RT. log. 





(The brackets indicate concentration in mols per liter.) Equili- 
brium will be established only when the product of the concen- 
trations of Nat and Cl’ has the same value on both sides of the 
membrane. 

Since this equation of products is of vital importance to the 
quantitative development of the theory here presented, any 
doubt as to its validity should be dispelled at the outset. The 
derivation of the equation need not involve the use of thermo- 
dynamics, because it can readily be visualized. In passing from 
one phase to the other, the oppositely charged ions must move in — 
pairs, since they would otherwise set up powerful electrostatic 
forces that would prevent their free diffusion. For this reason, 
a sodium or a chlorine ion striking the membrane alone could not 
pass through it. But, since the membrane is freely permeable to 
both Nat and Cl’, when two oppositely charged ions strike 
the membrane together, there is nothing to prevent them from 
passing through into the solution on the opposite side. The rate 
of transfer of these ions from one solution to the other depends, 
therefore, upon the frequency with which they chance to strike 
the membrane in pairs, which is measured by the product of their 


APPLICATION OF HETEROGENEOUS EQUILIBRIA 3 


concentrations. At equilibrium, the rate of transfer of Na+ and 
Cl’ from Solution II to Solution I exactly equals the rate of trans- 
fer of these ions from Solution I to Solution II, from which it 
follows that the product of the concentrations of these ions has 
the same value in both solutions. 

We may now consider the effect of adding another salt, such as 
KBr, to the system. Following the same line of reasoning, it will 
be evident that equilibrium will be established only when the 
product [K+] X [Br’] has the same value on both sides of the 
membrane and the same will be true for the products [Kt] X 
[Cl’] and [Nat] X [Br’]. In fact, with any number of mono- 
monovalent ionogens present in the system, the product of the 
concentrations of any pair of diffusible and oppositely charged 
ions will have the same value in both solutions. 

Introducing polyvalent ions into the system makes the equa- 
tion of products but very little more complicated. When a 
polyvalent ion strikes the membrane, it will pass through only 
when an equivalent number of ions of opposite sign strike the 
membrane at the same time and pass through with it. The rate 
of transfer of any dissociated ionogen from one solution to the 
other is evidently determined by the product of the concentrations 
of all of the ions required to produce the undissociated ionogen. 
At equilibrium, this product will have the same value in both 
solutions. If, for example, the system contained the ions Nat 
and SO,”’, then the product [Nat] X [Nat] X [SO.’’], or [Nat]? x 
[SO,4”], would have the same value on both sides of the membrane, 
at equilibrium. 

With the equation of products as a starting point, we may now 
consider further the nature of the unequal distribution of ions 
between the two solutions, caused by the impermeability of the 
membrane to the anion R’. In Solution II of the simple system 
including only the ionogens NaR and NaCl, let 

eee Nat p= 1Cl"] 
In Solution I let y = [Cl’] 
and z= |R’| 
whereupon Nae =.) 1-2 


The equation of products may then be written 
a? = yy + 2) 


4 COLLOIDAL BEHAVIOR 


But here we have the product of equals equated to the product of 
unequals, from which it is apparent, mathematically, that the 
sum of the unequals is greater than the sum of the equals, or that 


ZY ie Poe 


The reasoning thus indicates that the concentration of diffu- 
sible ions in Solution I, at equilibrium, is greater than in Solution 
II. If we let the excess of concentration of diffusible ions of 
Solution I over Solution II be represented by e, then 


Ayo 22ers 
and z=yt Vey 


which shows us further that 2 is greater than y, or that the con- 
centration of ionized sodium chloride is greater in Solution II 
than in Solution I. The added sodium chloride does not dis- 
tribute itself equally throughout both solutions, pu is the more 
concentrated in Solution II, at equilibrium. 

The different distribution of ions in the solutions at equilibrium 
gives rise, not only to a difference in osmotic pressure, but also 
to an electrical difference of potential across the membrane. 
Donnan derived the equation for this potential difference by the 
following thermodynamic reasoning. 

In the system just described, let 7, be the potential, for positive 
electricity, of Solution I, and z,, that for Solution II. Let the 
extremely small quantity Fdn of positive electricity be trans- 
ferred isothermally from II to I. In this virtual change of 
the system from equilibrium, the following work terms must be 
considered: The change in free electrical energy represented by 
Fdn(r, — 7) and the simultaneous transfer of pdn mols of Nat 
from II to I and of gdn mols of Cl’ from I to II, where p and q 
are the respective transport numbers of the ions, and, hence, p + 
g = 1. The maximum osmotic work of operation of this transfer 
of ions is represented by the expression 


[Nath 
[Na*], 


[Cl]; 
[CVn 








pdn.RT. log. + qdn.RT. log, 


But, since the system is in equilibrium, the electrical virtual 
work must balance the osmotic virtual work, or 


APPLICATION .OF HETEROGENEOUS EQUILIBRIA 5) 


[Nat] [Cl]; 
[Nat], [CV'lu 





Fdn(m — t,) = pdn.RT. log, 
[Nat]; — [Cl’l, 


™ 4 odn.RT. log. 








But [Nat] a5 tay = and Pp te vo 1 Letting yc = 7; — Tr, 
we have 
Jeak x 
eile ca loge volts 


This is an equation of fundamental importance in dealing quan- 
titatively with the electrical phenomena associated with colloidal 
behavior. 

It will now be shown that this equation is still valid when 
other ions of any valency are added to the system. Consider the 
general case where an lonogen yielding the ion M*+ of valency ais 
added. By applying the above line of reasoning to the potential 
difference produced by the unequal distribution of the ions of the 
added ionogen between Solution I and Solution II, we arrive at 
the equation 

edb [M*+],, 
Ong U8 Tutor, 





nF 


where n = a, the valency of M*+. But it is evident from the 
equation of products that 


[Me], x (Cl, = [Me]. X (Cl'l*n 
and that INat|?, < [Cl’]¢, = [Nat]¢, x [Cl]:, 
from which it is apparent that 

[Mo] hs [Nat]¢, Giiey 








Piste [Nat|*, . y° 
peak cia Pid atk x 
Therefore, R= a" log, jee loge 


This equation shows that, at equilibrium, the unequal distribu- 
tion of the added ionogen between Solution I and Solution II is 
producing exactly the same potential difference as the unequal 
distribution of sodium chloride. Although the addition of any 
ionogen must produce a change in the measured potential dif- 
ference, by disturbing the equilibrium, all ionogens present when 
equilibrium is again established are producing the same potential 
difference, regardless of valency. The potential difference can 


6 COLLOIDAL BEHAVIOR 


thus be calculated from the determination of the distribution of 
only one kind of ion between the two solutions. 

It should be constantly borne in mind that the complexity of 
the systems just described is due to the presence of the non- 
diffusible ion R’ in one of the phases. The equations, which 
have been derived by recourse only to orthodox physical chemis- 
try, permit one to calculate the effect of the presence of R’ upon 
the distribution of ions between the two phases and the concomi- 
tant osmotic pressure and difference of potential. 

The most complete proof of the correctness of Donnan’s theory 
of membrane equilibria has been furnished by Jacques Loeb,? who 
studied the equilibria resulting from the separation of solutions 
of protein salts from protein-free aqueous solutions by collodion 
membranes. In this short chapter, it would be impossible to 
give an adequate description of Loeb’s numerous and compre- 
hensive experiments; the reader should consult his book and the 
files of the Journal of General Physiology. (See also Chapter II 
of this book.) We must be content here with a brief description 
of only a portion of the work dealing with the Donnan theory. 

In his experiments, Loeb used collodion bags which were 
completely permeable to water and the simpler acids, bases, and 
salts, but not to dissolved proteins. In order to correlate Loeb’s 
experiments with the Donnan theory just described, let us 
consider, first of all, a solution of gelatin chloride and hydro- 
chloric acid contained in a collodion bag which is brought into 
contact with pure water. Hydrochloric acid diffuses out through 
the membrane until equilibrium is established between the 
external solution and the gelatin solution inside the bag. The 
outer solution contains only hydrochloric acid, but the inside 
solution contains both hydrochloric acid and gelatin chloride. 
At equilibrium, in the outer solution, let 


o = [Ht] Ser 


and in the inside solution let 


= [A 
and z = [gelatin ion*] - 
whence [CU res ae 


2Lors, Jacques: “Proteins and the Theory of Colloidal Behavior,” 
McGraw-Hill Book Co., New York, 1922. 


APPLICATION.OF HETEROGENEOUS EQUILIBRIA ve 


It is apparent from the reasoning given in connection with the 
Donnan theory that, at equilibrium, 


a? = yly + 2) 
and that 2 2 oe 


The greater concentration of diffusible ions of the inside solution, 
2y + z, must give rise to an osmotic pressure proportional to 
the quantity e in the expression 


eS 2y -- 2 — 2x 


The equations show that x must always be greater than y, or 
that the concentration of hydrogen ion is greater in the outer 
solution than in the collodion bag. In numerous experiments, 
Loeb showed that this is invariably true for acid solutions of 
gelatin and other proteins. It is also obvious that 2y + z is 
greater than 2z, or that the concentration of chloride ion is 
greater in the collodion bag than in the external solution, and 
this Loeb has also shown to be invariably true. The concentra- 
tions of hydrogen ion and chloride ion in both solutions were 
determined by means of hydrogen and calomel electrodes, 
respectively. This made it possible to test the validity of the 
fundamental equation of products. 

Table I gives the results of a series of experiments’ in which 
1 per cent solutions of gelatin chloride, acidified to different 
extents with HCl, were placed in collodion bags immersed in pure 
water. After 18 hours, equilibrium was established and values for 
[H*] and [Cl’] were determined both in the gelatin solutions and 


in the external, protein-free solutions. According to the equa- 
} 


La Ee 
tion of products, log Sry must equal log or where o and 1 


indicate concentrations in the outside and inside solutions, 
respectively. The results show that this equality holds, within 
the limits of experimental error, over the wide range of pH values 
from 4.04 to 1.73. 

Determinations of osmotic pressure were also found to be in 
harmony with the Donnan theory. At 24°C., the osmotic 
pressure, in terms of millimeters pressure of a column of water, 
equals 250,000e, where e is determined by the expression 2y + 


3Lors, Jacquns: J. Gen. Physiol., 3 (1921), 688. 


8 COLLOIDAL BEHAVIOR 











TasLE I 
pH or —log [H*]; log an log cai 
4.04 0.60 0.55 
3.92 0.62 0.60 
3.78 0.66 0.57 
3.61 0.55 0.50 
3.46 0.50 0.53 
3.16 0.43 0.38 
2.73 0.30 0.32 
2.36 0.20 0.47 
2.04 0.12 0.12 
lo7s 0.07 0.07 








z — 2x. For both gelatin and casein chlorides, Loeb found that 
the observed osmotic pressure approximated the values calculated 
from 250,000e as closely as the accuracy of the determinations 
of x, y, and z would permit. 

By a very ingenious arrangement, Loeb succeeded in measuring 
the membrane potentials predicted in Donnan’s theory. The 
apparatus used consisted essentially of a pair of saturated calomel 
electrodes, each having a capillary arm filled with a saturated 
solution of KCl. The end of one capillary arm was dipped into 
the gelatin solution in the collodion bag and the end of the other 
into the external solution. The calomel cells were then connected 
to a Compton electrometer and the potential difference measured. 
The potential difference measured was that of the cell 














o 
= 
calomel | saturated | external & gelatin | saturated calomel 
electrode KCl solution g solution KCl electrode 
=| 


Everything else being symmetrical, the potential difference 
measured was that between the external solution and the gelatin 
solution, across the collodion membrane. 

According to the Donnan theory, this potential difference is 
represented quantitatively by the expression 


bay x 
E = die lone 


APPLICATION .OF HETEROGENEOUS EQUILIBRIA 9 


As we have pointed out, regardless of the number of kinds of ions 
present or their valency, in calculating HL, we may let x represent 
the concentration of any monovalent ion in the external solution, 
and y its concentration in the protein solution. Since the con- 
centration of hydrogen ion lends itself readily to determination, 
it was selected by Loeb for the purpose. The validity of the 
equation is proved by the results in Table IJ. The measurements 
were made on 1 per cent solutions of gelatin chloride containing 
different amounts of HCl and enclosed in collodion bags immersed 
in pure solutions of HCl, each system being in equilibrium at the 
time the measurements were made. In the above equation, 
changing from natural to common logarithms and substituting 


the numerical value for ue at 25°C., we get 
E = 59 log * = 59 (log x — log y) millivolts 
where zx is the value of [H+] in the external solution and y its 


value in the gelatin solution. It is thus rendered possible to 
get the membrane potential by means of hydrogen electrode 








TaBLeE II 
pH value | Potential difference (millivolts) 
Gelatin External By hydrogen By Loeb’s 
solution solution electrode apparatus 








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10 COLLOIDAL BEHAVIOR 


measurements as well as by means of Loeb’s apparatus just 
described. Values obtained both ways are given in the table and 
their close agreement testifies to the correctness of the theory. 

We may next consider the effect of. adding neutral salts to 
the gelatin- HCl systems just discussed. Take the mono-monova- 
lent salt MN, neither of whose ions combine with the gelatin, and 
let its concentration in the external solution, at equilibrium, be 
represented by wu and in the protein solution by v. It is evident 
from the general equation of products that the product 


((H+] + [M+]) x ({Cl’] + [N’]) 
will have the same value in both solutions, or that 
(z + u)? = (y -b 0) (y eg) 
from which it is apparent that 
e=22y+v) +2—- 2¢+ 4) 
Solving the two preceding equations simultaneously, we get 
e= —2(¢+u) + VJ A(x + uw)? + 2? 


If the values for « and z are kept constant and the value of u 
increased without limit, itis obvious that the value of e approaches 
zero as a limit. In other words, by adding to the acid protein 
system a neutral salt which does not combine with the protein, 
we bring about a lowering of the value of e with its concomitant 
lowering of the osmotic pressure and potential difference between 
the two phases. It is important to recognize that it is the opera- 
tion of the distribution law and not any supposed repression of 
ionization of the protein salt that brings about the lowering of 
the value e. 

The correctness of this theory of the action of neutral salts is 
amply proved by Loeb’s experiments. He has also shown that 
the effect of valency in both acid and alkaline protein systems 
is quantitatively in accord with the theory we have described for 
membrane equilibria. Lack of space forces us to refer the reader 
directly to Loeb’s publications. 


JELLY EQUILIBRIA 


When Donnan first published his theory of membrane equi- 
libria, Procter,4 who was then studying the swelling of gelatin, 
4 Procter, H. R.: J. Chem. Soc., 105 (1914), 318. 


APPLICATION. OF HETEROGENEOUS EQUILIBRIA 11 


recognized that the same fundamental principles apply to the 
equilibrium between solid gelatin and dilute hydrochloric acid 
solutions, although jelly equilibria are somewhat more 
complicated. 

When a strip of dry gelatin is soaked in water below 30°C., 
it swells by absorbing water, increasing in volume from 500 to 
1,000 per cent, depending upon the temperature of the water and 
quality of the gelatin. With increasing concentration of acid 
or alkali, the swelling increases to a maximum and then decreases. 
The property of swelling in aqueous solutions appears to be 
common to all proteins under conditions such that they do not 
pass directly into solution. The swelling caused by acids and 
alkalies is generally counteracted by the addition of neutral 
salt or by increasing the concentration of acid or alkali sufficiently. 

While attempting to arrive at a rational explanation of the 
molecular mechanism of tanning, Procter was continually con- 
fronted by the necessity of first explaining the mechanism 
of the swelling of protein jellies. In 1897 he started an investi- 
gation® of the swelling of gelatin in solutions of acids and salts 
which has culminated in the present Procter-Wilson theory 
of swelling. 

Procter’s general method of experimentation was as follows: 
Sheets of thin, purified bone gelatin were cut into portions 
containing exactly 1 g. each of dry gelatin. A portion was put 
into each of a series of stoppered bottles containing 100 cc. of 
hydrochloric acid of definite concentration. After 48 hours, 
which was shown to be sufficient for the attainment of practical 
equilibrium, the remaining solution was drained off and titrated 
with standard alkali. The gelatin plates were quickly weighed 
and the volume of solution absorbed was calculated from the 
increase in weight of the plates. The swollen gelatin was then 
put back into the bottles and covered with enough dry sodium 
chloride to saturate the solution which had been absorbed by the 
gelatin. This caused the gelatin to contract and give up the 
absorbed solution. After 24 hours, when equilibrium was again 
established, the solution expelled by the salt was drained off 
and titrated to determine the amount of free acid which had been 


6 Procter, H. R.: Kolloidchem. Bethefte (1911); J. Am. Leather Chem. 
Assoc., 6 (1911), 270. 


12 COLLOIDAL BEHAVIOR 


absorbed by the gelatin. A small amount, usually about 1 cc., 
of solution always remained unexpelled by the salt and, although 
not strictly true, this was assumed to have the same concentra- 
tion of free acid as the portion expelled, due allowance being 
made for the increase in volume of solution due to saturating it 
with salt. The acid still unaccounted for was assumed to be 
combined with the gelatin base. 

A further set of checks was obtained by dissolving the gelatin, 
dehydrated by treatment with salt, in warm water and titrating 
with standard alkali, using both methyl orange and phenol- 
phthalein, the former indicating the free acid left in the jelly 
and the latter the total, including the acid combined with the 
gelatin base, which was obtained by difference. 

Procter’s experimental results will be considered in connection 
with the Procter-Wilson theory of swelling® now to be discussed. 
Instead of tracing the development of this theory from Procter’s 
earliest work to its present status, it will simplify matters to 
present the theory from the deductive reasoning furnished. 
later by Wilson and Wilson.? They set out to prove that the 
entire equilibria can be determined quantitatively from the 
orthodox laws of physical chemistry on the simple assumption 
that gelatin, or any other protein, combines with hydrochloric 
acid to form a highly ionizable chloride, or with any ionogen 
to form a highly ionizable protein salt. It seemed that success 
in this would furnish substantial proof of the correctness of 
the theory. It might be mentioned that Procter’s experiments 
led him very early to the view that gelatin and HCl combine, 
much in the same manner as do NH; and HCl, to form a highly 
ionizable chloride. 

In order to make the reasoning general, let us consider the 
hypothetical protein G, which is a jelly insoluble in water, 
is completely permeable to water and all dissolved ionogens 
considered, is elastic and under all conditions under consideration 
follows Hooke’s law, and combines chemically with the hydrogen 
ion, but not the anion, of the acid HA according to the equation 


[G] X [H+] = K[GH*] (1) 


° Procrur, H. R. and Winson, J. A.: J. Chem. Soc., 109 (1916), 307. 
" Witson, J. A. and W. H.: J. Am. Chem. Soc., 40 (1918), 886. 


APPLICATION OF HETEROGENEOUS EQUILIBRIA 13 


In other words, the compound GHA is completely ionized into 
GH* and A’. The brackets indicate concentration in mols per 
liter. 

Now take 1 millimol of G and immerse it in an aqueous 
solution of HA. The solution penetrates G, which thereupon 
combines with some of the hydrogen ions, removing them from 
solution, and, consequently, the solution within the jelly will have 
a greater concentration of A’ than of H*, while in the external 
solution [Ht] is necessarily equal to [A’]. The solution thus 
becomes separated into two phases, that within and that sur- 
rounding the jelly, and the ions of one phase must finally reach 
equilibrium with those of the other phase. 

At equilibrium, in the external solution, let 


a = [H*| = [A’ 
and in the jelly phase let 


Veeey ELT 
and e = (GH*| 
whence [A] =y +2 


It is apparent from the line of reasoning given earlier in the 
chapter that the product [H+] X [A’] will have the same value 
in both phases at equilibrium, or that: 


x? = y(y +2) (2) 
As was also pointed out above, it is evident from equation (2) that 
Gea 2a 2" 20 (3) 


where e is defined as the excess of concentration of diffusible ions 
of the jelly phase over that of the external solution. From (2) 
and (3) we get tha 

t=yt+ Vey (4) 


which shows that x is greater than y, or that the hydrogen ion 
concentration is greater in the external solution than in the 
jelly. This, in turn, shows that [A’] is greater in the jelly than 
in the external solution. 

Since [A’] is greater in the jelly than in the surrounding solu- 
tion, the anions of the protein salt will tend to diffuse outward 
into the external solution, but this they cannot do without 


14 COLLOIDAL BEHAVIOR 


dragging their protein cations with them. These protein cations, 
however, are not in solution in the generally accepted sense of 
the word “solution,” but form part of an elastic structure which 
resists the pull of the anions, which are actually in solution. 
The actual measure of the pull is that of the difference between 
' the total energy directed outward from the jelly and that directed 
from the external solution towards the jelly. This is obviously 
represented by the value ¢ and, according to Hooke’s law, 


ed 0074 (5) 


where C' is a constant corresponding to the bulk modulus of the 
protein and V is the increase in volume, in cubic centimeters, 
of 1 millimol of the protein. 

Since we have taken 1 millimol of G, 


1 
[G] a [IGH*) Sia (V +a) 
1 


where a is the initial volume of 1 millimol of the protein. 
From (1) and (6) 


2 ala SE 
and from (2) and (8) 
z2=e+ 2/ey (8) 
From (5) and (8) 
2=CV+2VCVy (9) 
and from (7) and (9) 
(V + a)(K + y)(CV + 2V/CVy) — y =0 (10) 


where the only variables are V and y. 

If the molecules or atoms of the protein are not themselves 
permeable to all ions considered, the quantity a should not be 
taken as the whole of the initial volume of the Jelly, but only as 
the free space within the original, dry jelly through which ions 
can pass. For our hypothetical protein, then, we shall consider 
the limiting case where the value of a is zero. This assumption 
in the case of gelatin introduces errors less than the probable 
experimental error because of the relatively large values for V 


APPLICATION OF HETEROGENEOUS EQUILIBRIA 15 


over the significant swelling range. Equation (10) thus reduces 
to | 


V(K + y)(CV + 24/CVy) —y = 0 (11) 


Knowing the values of the constants K and C, we can plot the 
entire equilibrium as a function of any one variable; given y, we 
ean calculate V from (11); given V, we can calculate e from (5); 
given y and e, we get z from (8); and we can then get x from (3). 

Procter and Wilson obtained the value K = 0.00015 for the 
sample of gelatin used in their experiments by adding successive 
portions of standard HCI to a dilute solution of the gelatin and 
noting the corresponding changes in hydrogen ion concentration 
by means of the hydrogen electrode. The difference between the 
concentration of hydrogen ion that would have been found upon 
adding the acid to pure water and that actually found by adding 
it to the same volume of gelatin solution was taken as the 
amount of acid combined with the gelatin, or as the value of [GH] 
in equation (1). Substituting any two sets of determinations of 
[GH*] and [H*] in equation (1) and solving the resulting equa- 
tions simultaneously, the value of K can be found. It is signifi- 
cant that the value for K was determined for gelatin in solution 
and independently of the swelling experiments. 

This left only one constant to be determined from the swelling 
experiments themselves. C was found by substituting experi- 
mental values for V and e in equation (5). It was found to vary 
with the temperature and with the quality of the gelatin, as 
would have been expected, but had the value 0.0003 for the 
gelatin used by Procter at 18°C., the temperature at which his 
experiments were made. 

In order to compare calculated values for V with experimental 
determinations of the increase in volume of 1 g. of gelatin, it is 
necessary to know its equivalent weight. Procter originally 
regarded gelatin as a diacid base with a molecular weight of 839, 
but later work by Procter and Wilson showed that it should 
rather be regarded as acting as a monacid base with an equivalent 
weight of 768, in acid solutions not sufficiently concentrated to 
cause decomposition. They found that 768 g. of gelatin combine 
with a limiting value of 1 mol of hydrochloric acid and the 
combination resembles that of HCl with a weak, monacid base. 


16 COLLOIDAL BEHAVIOR 


For this reason we may use the value 768 as the equivalent weight 
of gelatin. As for the molecular weight of gelatin, no convincing 
figures have yet been obtained and it may be questioned whether 
they would be of any great value anyway. We look upon a plate 
of gelatin as a continuous network of chains of amino acids, there 
being no individual molecules, unless one wishes to look upon the 
entire plate of gelatin as one huge molecule. 
By substituting the values K = 0.00015 and C = 0.0003 in 
equation (11), Wilson and Wilson were able to calculate all of 
the variables of the equilibrium for gelatin and HCl over the 
range covered by Procter’s experiments. These are given 
in Table III along with Procter’s actual determinations taken 


Taste II].—AtT Eaquiniprium 


















































Cubic centimeters | 
of solution Suh _ [Total chloride] 
aa [HCl] V absorbed by 1 g. (ACU are in jelly 
for Mi | Calcu- gelatin | 
Gane lated Seah Vinta cS 
/ Calcu- Ob- Calcu- | Ob- Calcu- Ob- 
| lated served | lated | served lated | served 
I 
0.006 0.0011 | 33.3 43.4 | 44.1 0.0001 | 0.0005 | 0.012 0.014 
0.008 0.0018 37.5 48.8 48.7 0.0002 | 0.0004 0.014 0.015 
0.010 0.0025 41.7 54.3 59.9 0.0004 | 0.0004 0.016 0.015 
0.010 0.0028 42-7 55.6 58.4 0.0004 | 0.0004 0.017 0.015 
0.010 0.0032 43.2 o6e2 98 67 0.0005 | 0.0005 0.019 0.017 
0.015 0.0073 40.8 on. Seo 0.0020 | 0.0020 0.024 0.020 
0.015 0.0077 40.2 52.3 5252 0.0020 | 0.0020 0.025 0.022 
0.015 0.0120 ofeo 48.8 51.9 0.0050 | 0.0060 0.031 0.027 
0.020 0.0122 aye Ge 48 .6 Bk 76 0.0050 | 0.0060 0.031 0.027 
0.025 0.0170 34.5 44.9 40.4 0.0080 | 0.0090 0.036 0.037 
0.025 0.0172 34.3 44.7 48.1 0.0080 | 0.0090 0.036 0.031 
0.050 0.0406 2627 34.8 36.4 0.0260 | 0.0300 0.063 0.061 
0.050 0.0420 26.4 34.4 Sil 0.0270 | 0.0300 0.065 0.068 
ete eis 0.0576 24.0 coy be 34.0 0.0410 | 0.0430 0.082 0.079 
0.075 0.0666 23.0 29.9 27.9 0.0490 | 0.0500 0.092 0.095 
0.075 0.0680 22.8 29.7 29.1 0.0500 | 0.0530 0.094 0.092 
0.100 0.0930 20.7 27.0 24st Sat 0.0720 | 0.0720 0.121 0.126 
0.100 0.0944 20:5 2620 26.4 0.0730 | 0.0720 On L227 0.121 
snares 0.1052 19.8 2528 29.8 0.0830 | 0.0850 0.134 0.128 
0.125 0.1180 18.9 24.6 24.4 0.0950 | 0.0900 0.148 0.148 
0.150 0.1434 17.9 2373 24.0 0.1180 | 0.1180 0.174 0.173 
0.150 0.14385 17.9 23.3. 24.2 0.1180 | 0.1180 0.174 0.172 
Ons 0.1685 UG ol 227G oie 0.1410 | 0.1380 0.200 0.200 
0.200 0.1925 16.3 a es 20.6 0.1640 | 0.1610 0.225 0.229 
0.200 0.1940 16.2 abe 2277, 0.1660 | 0.1650 0.227 0.225 
0.200 0.1945 1632 ae 2251 0.1670 | 0.1640 0.228 0.226 
0.250 0.2450 Lond 19.7 20.2 0.2130 | 0.2100 0.279 0.281 
0.300 0.2950 14.0 1822 20.0 0.2610 | 0.2600 07332 0.332 
| 








= eee 


~~. 


i EEE ae 


APPLICATION OF HETEROGENEOUS EQUILIBRIA 17 


from the table on page 317 of his 1914 paper. They are also 
shown graphically in Figs. 1 and 2, in which the concentration of 
gelatin chloride is taken as the difference between the concen- 
trations of total chloride and free hydrochloric acid in the jelly. 
It will be noted that the agreement between calculated and 
observed values is absolute, within the limits of experimental 


0:30 





Procter's observed results: 
% =total chloride 


0:25 e =free HCl 
° = gelatin chloride 


Continuous lines represent 
020 calculated values 


Concentration inJelly (mols per liter) 


0 0.05 0.10 0.15 0.20 0.25 030 
Mols Hydrogen Ion per Liter of External Solution at Equilibrium 


Fig. 1.—Observed and calculated values for the distribution of HCl in the 
system gelatin-HCl-water. 


error. For this reason the theory may be regarded as proved. 
It is worthy of note that no other theory of the swelling of jellies 
has yet passed the stage of qualitative speculation. 

According to the theory, all monobasic acids should produce 
the same degree of swelling of a given protein for any fixed 
hydrogen ion concentration, under constant conditions, provided 
the protein salts formed are ionized to the same extent. It was 
formerly thought that different monobasic acids produce different 


8 Procter, H. R.: J. Chem. Soc., 105 (1914), 313. 


18 COLLOIDAL BEHAVIOR 


degrees of swelling, following the order known as the Hofmeister 
series of the ions, but Loeb and Kunitz? showed that earlier 
investigators, through failure to measure the hydrogen ion con- 
centration, had fallen into the error of attributing to the several 
acids effects caused merely by differences in hydrogen ion con- 
centration. For a fixed value of x, they obtained the same degree 


Continuous line represents calculated valves 
Crosses represent Procter& observed resu/ts 








Increase in Volurne of 1 Gram of Gelatin (cubic centimeters) 





0 
0 0.05 0.10 OS 0.20 0.25 0.30 
Mols Hydrogen Ion per Liter of External Solution at Equilibrium 
Fig. 2.—Observed and calculated values for he degree of swelling of gelatin 


as a function of the concentration of hydrochloric acid in the external solution 
at equilibrium. 


of swelling of gelatin with all monobasic acids studied as well as 
with such acids as phosphoric and oxalic at concentrations at 
which they act as monobasic. 


The extent of swelling of proteins by polybasic acids, which . 


combine as such with the protein, will, according to the theory, 
be considerably less than that caused by monobasic acids at the 
same hydrogen ion concentration, because a smaller total number 
of anions will be associated with equivalent weights of the protein. 


® Lorn, J. and Kunitz, M.: J. Gen. Physiol., 5 (1923), 665. 


APPLICATION OF HETEROGENEOUS EQUILIBRIA 19 


For example, for equivalent weights of gelatin sulfate and gelatin 
chloride, there would be only half as many sulfate as chloride ions. 
For very small values of x, we should, therefore, expect sulfuric 
acid to produce only half the degree of swelling produced by 
hydrochloric acid at the same value of x, and Loeb has repeatedly 
shown this to be true. 

It will be apparent from the discussion of membrane equilibria 
that the value of e in the case of acid-swollen jellies also will be 
lowered by the addition of neutral salts which do not combine 
with the protein. This means a decrease in swelling, which is in 
perfect harmony with all experimental data. 

It is also apparent that there must be a measurable difference 
of potential between the jelly phase and the external solution; this 
has actually been measured by Loeb and found to be in quantita- 
tive agreement with the theory. 


SURFACE EQUILIBRIA 


It was pointed out by the present writer! in 1916 that at the 
surface of contact between an aqueous solution and any electric- 
ally charged solid there must exist an equilibrium of the type just 
described for jellies. As an example, we may consider a gold sol. 
As has been shown by Beans and Eastlack,!! when gold is dis- 
persed in water, the presence of chloride, bromide, iodide, or 
hydroxide ion in concentrations ranging from 0.00005 to 0.005 
normal has a marked stabilizing effect on the sol produced and 
the particles are negatively charged. The effect seems to be 
due to the ability of these ions to form stable compounds with 
the gold. Fluoride, nitrate, sulfate, and chlorate ions decrease 
the stability of gold sols, which is significant in view of the fact 
that they do not form stable compounds with gold. 

Let us consider any single particle of gold in a sol stabilized 
with potassium chloride. In combining with the gold, the chlo- 
ride ions have imparted their negative charges to the particle. 
But the potassium ions are still left in solution, although their 
field of motion is restricted to the thin film of solution wetting 

10 Witson, J. A.: J. Am. Chem. Soc., 38 (1916), 1982. 

11 Beans, H. T. and Hastuack, H. E.: J. Am. Chem. Soc., 37 (1915), 
2667. 


20 COLLOIDAL BEHAVIOR 


the particies, because they must continue to balance the negative 
charges on the particle. The volume of the film of aqueous 
solution enveloping the particle will be measured by the surface 
area of the particle and the average distance that the potassium 
ions are free to travel from the oppositely charged surface. 
The enveloping film of solution will contain more potassium ions 
than chloride ions, since it contains ionized potassium chloride 
as well as potassium ions balanced only by the negative charges 
on the surface of the gold particle. In the solution far removed 
from the surface of the particle, potassium and chloride ions will 
be present in equal numbers. | 

The distribution of ions here described shows a striking 
analogy to the distribution of ions between a jelly and the 
surrounding solution. In the case of the jelly, the system 
consisted of the following three phases: the solid protein network, 
the solution between the interstices of the network, and the 
solution surrounding the jelly mass. In the case of the gold sol, 
we also have three phases: the solid surface, the film of solution 
wetting the surface, and the great bulk of solution further 
removed from the surface of the particle. 

To show the analogy still more clearly, let us adopt a similar 
system of notation and in the bulk of solution of the gold sol let 


xy = (Kt) = [Cle 
In the enveloping film of solution wetting the particles, let 


yy 
and z = [K*], balancing the charges on the 
particles, whence y + 2 represents the total concentration of 
potassium ion. 
It is immediately apparent that our equation of products 
applies to the distribution of ions between the bulk of solution 
and that portion wetting the particles, or that 


a? = y(y + 2) 


The surface film of solution will have a greater total concentra- 
tion of ions than the surrounding solution by the amount 2y + 
z — 2x. This unequal distribution of ions will give rise to an 


APPLICATION OF HETEROGENEOUS EQUILIBRIA 21 


electrical difference of potential between the enveloping film 
and the surrounding solution whose measure is 


ga kt, z_kr ci ied el 
% Ben la Aah 52 


F PF 
But now, if we increase the value of x without limit, while z 
remains constant, H must decrease, approaching zero as a limit, 
since 





eae py OT 20 
Be ay 08g = O 


It is thus evident that the difference of potential between the 
enveloping film and the surrounding solution will be a maximum 
when there is no free potassium chloride present and will decrease, 
approaching zero, as the concentration of potassium chloride is 
increased without limit. The writer believes that the electro- 
static repulsion opposing the coalescence of the particles is 
determined by this potential difference rather than by the 
absolute electrical charge on the particles because the surface 
film completely envelops the particles and tends to endow them 
with its own properties. 

When enough potassium chloride has been added to lower the 
potential difference to a point where it is no longer able to 
overcome the attractive forces between the particles and the 
surface tension of the enveloping film, the particles move toward 
each other and the enveloping films of two or more particles 
blend into one. It is at this point that the actual charges 
themselves come into play and probably determine the nature of 
the precipitate. It should be noted that the lowering of the 
potential difference between the enveloping film and surrounding 
solution does not necessarily involve any lowering whatsoever of 
the value of the electrical charge on the particle itself. It is also 
apparent that the theory holds just as well regardless of the 
actual cause of the charge on the particles. We have thus 
an explanation of the precipitation of sols by the addition of 
electrolyte. 

Discussion of the theory of heterogeneous equilibria presented 
in this chapter might be continued indefinitely, but it is hoped 
that enough has been given to show that the well-known distribu- 


22 COLLOIDAL BEHAVIOR 


tion law may become a powerful tool in clearing up many of the 
mysteries of colloid chemistry, once the phase boundaries in the 
systems studied are clearly recognized. Further discussions of 
many of the points presented in this chapter may be found in 
the recent books of Loeb,!* Bogue,!* Procter,'4 and the writer. 


2 Lons, JAcquEs: ‘‘Proteins and the Theory of Colloidal Behavior,” 
McGraw-Hill Book Co., New York, 1922. 

13 Boaur, RopertT Herman: ‘‘Chemistry and Technology of Gelatin and 
Glue,” McGraw-Hill Book Co., New York, 1922. 

4 ProcTER, Henry Ricwarpson: “Principles of Leather Manufacture,” 
D. Van Nostrand Co., New York, 1922. 

* Witson, JouHN Arruur: ‘The Chemistry of Leather Manufacture,”’ 
The Chemical Catalog Co., New York, 1923. 


CHAPTER II 


CRYSTALLOIDAL AND COLLOIDAL BEHAVIOR OF 
PROTEINS 


By 


JACQUES LOEB 


THE CRYSTALLOIDAL BEHAVIOR OF PROTEINS 


Chemical Behavior.—The treatment of the subject of colloidal 
behavior has in many cases not yet risen above the level of mere 
qualitative speculations. This is chiefly due to the fact that 
many of the authors of colloidal literature have failed to measure 
and to consider one of the main variables in their experiments, 
namely, the hydrogen ion concentration of their solutions or gels. 
If this quantity is duly measured and taken into consideration, the 
subject of colloidal behavior can be raised above the mere qualita- 
tive state and based upon a quantitative basis, which permits 
the derivation of the observed results from a rationalistic formula 
without the use of arbitrary constants. It is the intention of this 
article to show that this is true at least for one group of colloids, the 
proteins, the colloidal behavior of which can be explained quanti- 
tatively from Donnan’s theory of so-called membrane equilibria. 


1The following books are recommended for general reference to the 
physical chemistry of proteins: 
Micwaeg.is, L.: “Die Wasserstoffionenkonzentration,” Ist ed. Berlin, 
1914; 2nd ed., vol. 1, Berlin, 1922. 
Ropertson, T. B.: “The Physical Chemistry of the Proteins,’’? New 
York, 1918. 
Sorensen, 8. P. L.: “Studies on Proteins,” Compt.-rend. trav. lab. 
Carlsberg, vol. 12, Copenhagen, 1915-17. 
Pauut, W.: ‘Colloid Chemistry of Proteins,’’ New York, 1922. 
Boaugz, R. H.: “Chemistry and Technology of Gelatin and Glue,” 
New York, 1922. 
Witson, J. A.: “The Chemistry of Leather Manufacture,’’ New York, 
1923. . 
Logs, J.: ‘‘Proteins and the Theory of Colloidal Behavior,’’ New York, 
1922. (Referred to hereafter briefly as ‘‘ Proteins.’’) 
23 


24 COLLOIDAL BEHAVIOR 


The distinction between crystalloids and colloids was proposed 
by Graham in 1861, the crystalloids being characterized by a 
tendency to form crystals when separating from an aqueous solu- 
tion, and the colloids by a tendency to separate out in the form of 
‘“‘velatinous”’ (or amorphous) masses. Graham found that these 
two groups of substances differ also in other respects, first, in 
their ‘‘diffusive mobility,’’ and, second, in a peculiar “physical 
aggregation.” The crystalloids diffuse readily through many 
kinds of membranes (e.g., pig’s bladder, parchment) through 
which colloids can diffuse not at all or only very slowly. The 
second peculiarity is the tendency of the colloids to form aggre- 
gates when in solution, while this property is lacking or less 
pronounced in crystalloids. A brief quotation from a paper by 
Graham will illustrate these definitions: 


Among the latter [z.e., the substances with low order of diffusibility] 
are hydrated silicic acid, hydrated alumina, and other metallic per- 
oxides of the aluminous class, when they exist in the soluble form; 
and starch, dextrin and the gums, caramel, tannin, albumen, gelatin, 
vegetable and animal extractive matters. Low diffusibility is not the 
only property which the bodies last enumerated possess in common. 
They are distinguished by the gelatinous character of their hydrates. 
Although often largely soluble in water, they are held in solution by a 
most feeble force. They appear singularly inert in the capacity of 
acids and bases, and in all the ordinary chemical relations. But, on 
the other hand, their peculiar physical aggregation with the chemical 
indifference referred to, appears to be required in substances that 
can intervene in the organic processes of life. The plastic elements of 
the animal body are found in this class. As gelatin appears to be its 
type, it is proposed to designate substances of the class as colloids, 
and to speak of their peculiar form of aggregation as the colloidal 
condition of matter. Opposed to the colloidal is the crystalline condition. 
Substances affecting the latter form will be classed as crystalloids. The 
distinction is no doubt one of intimate molecular constitution.” 


It is, therefore, obvious that there are, according to Graham, at 
least two essential differences between colloids and crystalloids, 
the difference in diffusion through membranes, and the difference 
in the tendency to form aggregates in solutions. Both prop- 
erties play a réle in colloidal behavior, but they determine 


?Granam, T.: Phil. Trans. (1861), 183-224. Reprinted in “Chemical 
and Physical Researches,’’ p. 553, Edinburgh, 1876. ) 


BEHAVIOR OF PROTEINS 25 


different colloidal properties, as will be shown in the case of 
proteins. 

Recent investigations on proteins have yielded the result that 
the distinction between crystalloids and colloids is no longer 
tenable. It was found that proteins behave like crystalloids in 
regard to chemical combination, solubility, cohesion, and possibly 
some other properties; and that they show colloidal behavior 
only under one well-defined condition, namely, when the large 
protein ions are prevented from diffusing through membranes or 
gels which are readily permeable to the small ions of ordinary 
salts.* 

It is known through the work of Emil Fischer that the proteins 
are built up from amino acids in peptide linkage. The amino 
acids are amphoteric electrolytes and true crystalloids, being 
able to diffuse freely through dialyzing membranes such as 
parchment, and showing true solubility in water. Like the 
amino acids, proteins are also amphoteric electrolytes capable of 
combining both with acids and alkalies and forming true salts 
which undergo electrolytic dissociation.t Whether proteins 
combine with acids or with alkalies depends on the hydrogen ion 
concentration. At a certain hydrogen ion concentration, which 
varies with each individual protein (according to the amino acids 
of which it is composed), the protein combines with neither acid 
nor alkali, and this critical hydrogen ion concentration is called 
the isoelectric point. When the hydrogen ion concentration of 
a protein solution is greater than that of the isoelectric point, 7.e., 
when the solution is on the acid side from the isoelectric point, 
the protein forms salts of the type of protein chloride, protein 
sulfate, etc., while, when the hydrogen ion concentration is less 
than that of the isoelectric point, the protein forms salts of the 
type of metal proteinates, such as sodium proteinate, calcium 
proteinate, and so on. In the following the hydrogen ion con- 
centration of protein solutions or protein gels is expressed in terms 

3 Lous, J.: “‘Proteins,’”’ New York, 1922, and a series of articles in the 
Journal of General Physiology, vols. 1 to 5. 

4 Bucarszxy, 8S. and LirperMann, L.: Arch. ges. Phystol., T2 (1898), 51; 
Rosertson, T. B.: ‘The Physical Chemistry of the Proteins,’ New York, 
1918; Harpy, W. B.: J. Physiol., 33 (1905-06), 251; Pauit, W.: Fortschr. 


naturwiss. Forschung, 4 (1912), 223; ‘‘Kolloidechemie der Eiweissk6rper,” 
Dresden and Leipzig, 1920; Lous, J.; “Proteins,” New York, 1922, 


COLLOIDAL BEHAVIOR 


26 


of Sdrensen’s logarithmic symbol, pH (pH being the negative 


logarithm of the hydrogen ion concentration). 


A protein solution or gel is not adequately defined unless its 


pH has been measured. When a salt, e.g., AgNOs, is added to 


"04-3897 FORO JO pwoy OY} 4% poyxreUr st UOTN]OS uryejes Yoo Jo Ad ou, ‘Iwsx 
8 IOAO 10} 44ST] 02 posodxe ySnoy} svopo AyuUouvUTIOd poureulol sso] 10 JF A jo uneyes oy} o7tuM 
yep sea 2°>< Ad jo unjejes oy} snoy ue Jey ynoqe uy = +3431] 09 pesodxo puv seqn}-1s09 0}UI poanod 
UoY} O10M SUOTNIOS OY], “peululiojyep sem FFA oy} puv ‘uoNjos yueo red [ & 04 yy sno01q ‘payenbyy 
SEM Ul}EIOS ONT, “UNVIOS YIM UOTeUIqUIOD UI YoU JOATIS ey} eAOUTOI 0} 10J8M poo YIM poysem 
eq} puv *ONSV 79/ W YiIM WOOL HIep & UI po}very sum Fd qUSIBQYIp 04 YYsSNoI1qG unees poropMog 
“JUIOd dO11}00]90sI OY} JO Opis oUTTey]Te oY} UO ATUO sUIO}OId YM SUIqUIOD SUOT}BO 4RT} joolg—T “DIY 


finely powdered gelatin at different pH, it is found that Ag gela- 


tinate can be formed only when the pH is greater than 4.7 (this 
being the isoelectric point of gelatin); and when K,Fe(CN). is 


BEHAVIOR OF PROTEINS 27 


added, gelatin ferrocyanide can be formed only when the pH is 
less than 4.7 (Figs. 1 and 2). This can be shown by methods 
described in a recent book.® 


All the samples of gelatin solution of pH<4.7 turned blue—in 


this: figure indicated in black—(through the formation of some ferric salt), while all the gelatin 


Fic. 2.—Proof that anions combine with proteins only on the acid side of the isoelectric point. 
solutions of pH 4.7 or above remained colorless. 


Doses of powdered gelatin solutions of different pH were treated with M/128 K4Fe(CN). and 





then washed with cold water. 


The proof that proteins combine stoichiometrically with acids 
and alkalies can be furnished by titration and combination curves. 
For this purpose (and perhaps for work with proteins in general) 
it is necessary to use, as standard material, protein of the pH 

5 Lous, J.: “‘Proteins,”’ p. 28. 


28 COLLOIDAL BEHAVIOR 


of the isoelectric point. We have seen that proteins combine 
with acids only at a pH below that of the isoelectric point, which 
for gelatin or casein is about pH 4.7, and for crystalline egg albu- 


20 
















Ce. O.1N acid in 100c.¢.1%lo solution of isoelectric albumin 
co 


-EEEEHES BR NEE 
oo 
ECCECEE EEE SSS SN 


"36 me Chee Wee aCe Hf 36 38 40 42 44 46 48 
p 






Fia. 3.—The ordinates represent the number of cubic centimeters of 0.1 N HCl, 
H2SOu, oxalic, and phosphoric acids required to bring 1 g. of isoelectric crystal- 
line egg albumin to the pH indicated on the axis of abscisse. Enough H2O 
was added to bring the albumin and acid to a volume of 100 cc. For the same 
pH the ordinates for HCl, H2SO.s, and phosphoric acid are approximately as 
1:1:3. The ratio of HCl to oxalic acid is a little less than 1:2, when pH is >3.0. 


min 4.8. It happens that at a pH below 4.7 most of the weak 
dibasic and tribasic acids dissociate as monobasic acids, Thus, 


BEHAVIOR OF PROTEINS 29 


H;PO, dissociates into H+ and the monovalent anion H.PO,. 
Hence, if acids combine stoichiometrically with isoelectric protein, 
it should require exactly three times as many cubic centimeters 
of 0.1 Nn H3PO, to bring a 1 per cent solution of an isoelectric 
protein, e.g., gelatin or crystalline egg albumin or casein, to the 
same hydrogen ion concentration, e.g., pH 3.0, as it requires of 
0.1 n HCl or HNO3. ‘Titration experiments show that this is 
the case. Furthermore, since H2SO, is a strong acid, splitting off 
both hydrogen ions even at a pH below 4.7, the same number of 
cubic centimeters of 0.1 N H2SO,z as of HCl should be required to 
bring 1 g. of isoelectric protein in 100 cc. of solution to the same 
pH, e.g., 3.0, and this was found also to be true. 

Figure 3 gives the titration curves for crystalline egg albumin 
for four acids, HCl, H2SO.1, H3;PQO,, and oxalic acid. One gram 
of isoelectric albumin was dissolved in 100 ec. of H.O containing 
varying amounts of 0.1 N acid. These cubic centimeters of 
0.1 Nn acid in 100 cc. solution are the ordinates of the curves 
in Fig. 3. The abscisse are the pH to which the protein solution 
was brought by the addition of acid. It takes always exactly 
three times as many cubic centimeters of 0.1 Nn H3PO, as it takes 
cubic centimeters of 0.1 N HCl or H.SO, to bring 1 g. of isoelectric 
albumin in 100 cc. of solution to the same pH. In order to bring 
the 1 per cent solution of originally isoelectric albumin to pH 
3.2, 9 cc. of 0.1 Nn HCI or H2SO, and 15 cc. of 0.1 n HsPO, must 
be contained in 100 cc. of the solution. To bring the albumin to 
pH 3.4, 4 ce. of 0.1 n HCl or H2SO, and 12 cc. of 0.1 n H3PO4 must 
be contained in the solution, and so on. 

Oxalic acid is, according to Hildebrand, a monobasic acid at a 
pH of 3.0 or below, but begins to split off the second hydrogen ion 
in increasing proportion above pH 3.0. ‘The titration curves 
show that about twice as many cubic centimeters of 0.1 N oxalic 
acid as 0.1 N HCl are required to bring the 1 per cent solution 
of isoelectric albumin to the same pH below 3.0, while it takes less 
than twice as many cubic centimeters of 0.1 N oxalic acid as 0.1 
n HCl to bring the albumin solution to the same pH if the pH 
is above 3.0. 

All this in itself would not yet prove that proteins combine 
- stoichiometrically with acids and alkalies, but this proof is 
furnished if we calculate the amount of acid in actual combina- 


30 COLLOIDAL BEHAVIOR 


tion with a given mass of protein. From the titration curves, 
the amount of acid in combination with 1 g. of originally isoelec- 
tric protein, ¢.g., crystalline egg albumin, in a 1 per cent solution 
of this protein at different pH can easily be calculated. Let us 
assume the acid added to isoelectric albumin to be HCl. If, 
e.g., at pH 3.0, 6 ce. of 0.1 N HCl are contained in 100 ce. of the 
1 per cent solution of the originally isoelectric albumin (as indi- 
cated in Fig. 3), part of the acid is in combination with the 
albumin and part is free. .How much is free is known from 
the pH of the albumin chloride solution, namely, 1 cc., since in 
the example selected the pH is 3.0 (Fig. 3). 

If 1 ce. is deducted from 6 cc., it is found that, at pH 3.0, 5 ce. 
of 0.1 n HCl are in combination with 1 g. of originally isoelectric 
crystalline egg albumin in 100cc. solution. A curveisconstructed 
in which the abscissz are the pH while the ordinates are the cubic 
centimeters of free 0.1 N HCl contained in 100 ce. of an aqueous 
solution as expressed by the pH of the solution. If the ordinates 
of this latter curve are deducted from the ordinates of the titra- 
tion curve in Fig. 3, we get a curve the ordinates of which give the 
number of cubic centimeters of 0.1 nN HCl in actual combination 
with 1 g. of originally isoelectric albumin in 100 cc. of solution. 

The results in Table I show the actual numbers of cubic centi- 
meters of 0.1 N solutions of each of the four acids in combination 
with 1 g. of originally isoelectric crystalline egg albumin in 100 
cc. of solution. The values for HCl and H,SO, are identical. 
Those for HsPO, are within the limits of the accuracy of the 
measurements, always three times as large as those for HCl. 

In the case of oxalic acid, we notice that at pH above 3.6 the 
number of cubic centimeters of 0.1 N oxalic acid in combination 
with 1 g. of albumin is less than twice that of HCl and that 
the difference is greater the higher the pH. At pH 3.2 and 
below, practically twice as many cubic centimeters of oxalic 
acid are, at the same pH, in combination with 1 g. of originally 
isoelectric albumin as there are of HCl. These titration experi- 
ments then leave no doubt that 1 g. of originally isoelectric albumin 
binds the same number of H ions at a given pH regardless of 
whether the acid added is a moderately weak acid like H;PO, or 
oxalic acid or a strong acid like HCl or H.SO,4. That these 


‘ Lozs, J.: “Proteins,” p. 44; J. Gen. Physiol., 1 (1918-19), 559; 3 (1920- 
21), 85. 


BEHAVIOR OF PROTEINS 31 


TABLE I.—Cusic CENTIMETERS OF 0.1 N ActIp IN CoMBINATION WITH 1 G. 
OF ORIGINALLY ISOELECTRIC CRYSTALLINE Eaqa ALBUMIN IN 100 Cc. 
oF SOLUTION 














pH HCl, H.SO,, Oxalic acid, H;PQ,, 

cc. CC. cc. cc. 
4.2 1 i Bs) 1.15 1.8 | 3.8 
4.0 . a0 1.70 2.6 5.3 
3.8 2.30 2.30 3.7 6.8 
3.6 2.90 2.90 5.0 8.6 
3.4 3.50 3.50 6.3 10.6 
3.2 4.20 4.30 8.0 13.1 
3.0 5.00 5.10 9.5 16.1 
2.8 5.80 5.90 aes 19.3 
2.6 6.70 6.50 13.3 22.9 
2.4 7.60 7.00 16.0 








simple facts had not been discovered earlier is the consequence of 
the failure of the workers to measure the hydrogen ion concentra- 
tion of their solutions. Had this been done, nobody would have 
thought of suggesting that acids combine with proteins according 
to the adsorption formula. 

The same proof was furnished by the writer for the combination 
of gelatin and casein’ and by Hitchcock for edestin and serum 
globulin. It shows that no matter whether a weak or strong 
acid is added to bring a given mass of isoelectric gelatin to the 
same pH, the same number of hydrogen ions is in combination 
with the protein. This is the expression of a purely stoichiomet- 
rical behavior and the simplest assumption is that the hydrogen 
ion of the acid is bound chemically by the protein. 

It can be shown with the aid of titration curves that isoelectric 
albumin combines with alkalies in the same stoichiometrical way 
as any acid, e.g., acetic acid, would combine with the same 
alkalies. If the cubic centimeters of 0.1 n KOH, NaO8H, 
Ca(OH)., or Ba(OH),: in 100 ce. of solution required to bring a 1 
per cent solution of isoelectric protein to the same pH are plotted 
as ordinates over the pH of the protein solution as abscisse, it is 


7 Lous, J.: “‘ Proteins.” 
8 Hircucock, D. I.: J. Gen. Physiol., 4 (1921-22), 597; 5 (1922-23), 35. 


32 COLLOIDAL BEHAVIOR 


found that the values for all four alkalies fall on one curve as 
they should if the combination occurred strictly stoichiometrically. 

The stoichiometrical character of the combination of proteins 
with hydrochloric acid can also be demonstrated by measuring 
the chlorine ion concentration of the solutions of protein chloride. 
When HCl is added to NH; (according to Werner) the H ions 
of the HCl are attracted to the nitrogen of the ammonia, while 
the Cl ions remain unaltered. The same type of reaction occurs 
when HCl is added to a solution of isoelectric gelatin. This was 
proved by measurements of the pCl of solutions of gelatin chloride. 
Different numbers of cubic centimeters of 0.1 N HCl were contained 
in 100 cc. of 1 per cent solutions of originally isoelectric gelatin 
and the pH and pCl of the solutions were measured, the pH with 
the hydrogen electrode and the pCl with the calomel electrode. 
It was found that the pCl was the same as if no gelatin had been 
present, while the pH was, of course, higher, thus showing that 
part of the hydrogen combines with the NH», and NH groups of 
the protein molecule while the Cl remains free (Table II).° 




















Tas_LeE II 
Cubic centi- Solution containing Solution containing 1 g. of 
meters of no gelatin isoelectric gelatin in 100 cc. 
0.1 n HCl in 
100 ce. 
solution pH pCl pH pCl 
| | 
2 212 2.72 4.20 2.680 
3 2.52 2.54 4.00 2.530 
4 2.41 2.39 
5 2.31 2.29 3.60 2.330 
6 2.24 2.26 3.41 2.250 
ff 2kG 2.18 3,20 2.180 
8 Bee 2.12 3.07 2.110 
10 zeur 2.01 2.78 2.025 
15 1.85 1.85 2.30 1.845 
20 1.72 | LG 2.06 1.760 
30 1.55 1.59 1.78 1.600 
40 1.48 1.47 1.61 1.470 





9 Lozs, J.: ‘‘Proteins,” p. 42. 


BEHAVIOR OF PROTEINS 30 


Hitchcock” has obtained similar results with crystalline egg albu- 
min, edestin, casein, and serum globulin, by using a silver-silver 
chloride electrode, so that it is possible to state that these results are 
true for many if not all proteins. ‘These measurements show that 
a protein chloride dissociates electrolytically into a large protein 
cation and a number of chlorine ions. A similar conclusion had 
already been reached by Manabe and Matula,!! and by Pauli’? 
on the basis of measurements of the pCl of gelatin and serum 
albumin chloride solutions. : 

The titration curves prove another fact, namely, that the salts 
of proteins are strongly hydrolyzed. When we add acid, e.g., 
HCl, to isoelectric protein, part of the acid combines with the 
protein, giving rise to protein chloride, while the rest of the acid 
remains free. There is then an equilibrium between free HCl, 
protein chloride, and non-ionogenic (or isoelectric) protein. The 
more acid is added to originally isoelectric protein, the more 
protein chloride is formed, until finally all the protein exists 
in the form of protein chloride. This point is reached for gelatin 
chloride at pH 2.5. It is possible to find out from the pH 
measurements how much of the acid added is free, and by 
deducting this value we know how much is in combination with 
the protein. By saturating the protein with acid, the combining 
weight of a protein with acid can be found. Hitchcock'® found 
in this way that the combining weight of gelatin is about 1,120. 
‘According to Dakin’s'‘ recent analyses, gelatin contains 1.4 per 
cent phenylalanine. Since 1 molecule of gelatin cannot contain 
less than 1 molecule of phenylalanine, and since the molecular 
weight of this amino acid is 165, the lowest possible weight 
of gelatin is 11,800. On this basis 1 molecule of gelatin 
should combine with 10 or a multiple of 10 hydrogen ions. 

Cohn and Hendry™ calculated from the titration curve of 
casein with sodium hydroxide a combining weight of about 2,100 
for casein. Dakin found about 1.7 per cent of tryptophane in 


10 Hircucock, D. I.: J. Gen. Physiol., 5 (1922-23), 383. 

11 Manase, K. and Marvuta, J.: Biochem. Z., 52 (1913), 369. 

122 Pau, W.: ‘Colloid Chemistry of Proteins,’’ New York, 1922. 

13 Hrrcucock, D. I.: J. Gen. Physiol., 6 (1923-24), 95. 

14 Dakin, H. D.: J. Biol. Chem., 44 (1920), 499. 

18 Coun, E. J. and Henpry, J. L.: J. Gen. Physiol., 5 (1922-23), 548. 


34 COLLOIDAL BEHAVIOR 


casein and, according to this, the molecular weight of casein 
must be 12,000 (or a multiple thereof). This would indicate 
that 6 or a multiple of 6 hydroxyl ions combine with 1 molecule 
of casein. Cohn and Hendry point out that the calculation of 
the molecular weight of casein from sulfur and phosphorus con- 
tents agrees also with the molecular weight estimated from the 
titration curves. 


Minimal molecular weight of casein calculated from tryptophane 


(aVeETAGE). oo. sc kc aes nes oe bata se 0s Seen te 12,800 
Minimal molecular weight of casein calculated from phosphorus X 3 13,116 
Minimal molecular weight of casein calculated from sulfur X 3..... 12,654 
Equivalent combining weight of casein for sodium hydroxide X 6.. 12,600 
AVOLOGZC! ee ce a's ee on mn oes wae 0 tee 8m eee 12,792 


All these data are difficult to understand on any other assump- 
tion than that proteins combine stoichiometrically with acid and 
alkali and that we are dealing with true chemical combination. 

A new proof that the combination of proteins with acids is true 
chemical combination, following the ordinary laws of classical 
chemistry, has recently been added by Hitchcock.'® Deamin- 
ized gelatin was prepared by treating gelatin with nitrous acid, 
following the procedure of Skraup. Determinations were made 
of the total nitrogen in gelatin and in deaminized gelatin, by the 
Kjeldahl method, and of the amino nitrogen in gelatin, by the 
Van Slyke method. It was found that the loss of total nitrogen 


in gelatin which had been deaminized by Skraup’s method was’ 


greater than the amino nitrogen originally present in the gelatin. 
Accordingly, the procedure of Skraup was modified by avoiding 
the application of heat in preparing the deaminized gelatin. 
The resulting product was found to have undergone a loss in 
total nitrogen exactly equal to the amino nitrogen originally 
present, indicating that under these conditions the deaminizing 
reaction really consisted simply in the replacement of amino 
groups by hydroxyl groups. 

In order to determine the combining capacity of deaminized 
gelatin for hydrochloric acid, it was necessary to ascertain its 
isoelectric point. This was done by measurements of the osmotic 
pressure developed at different pH values, following the proce- 
dure used by Loeb with other proteins. ‘The minimum of osmotic 


16 Hivcucock, D. I.: J. Gen. Physiol., 6 (1923-24), 95. 


a i el 


BEHAVIOR OF PROTEINS 30 


pressure, and, hence, the isoelectric point of the: protein, was 
found to be at pH 4.0. 

Finally, the combining capacity of the protein for hydrochloric 
~ acid was determined by electrometric titration with the hydrogen 
electrode, following a procedure similar to that previously used 
with gelatin and other proteins. It was found that the difference 
between the maximum combining capacities of gelatin and of 
deaminized gelatin for hydrochloric acid was approximately 
equivalent to the free amino nitrogen originally present in the 
gelatin and removed in the deaminizing reaction. ‘Thus the 
work constitutes a new type of evidence that the reactions of 
proteins with acid are truly chemical and stoichiometric. 

Solubility of Proteins.—It has been generally assumed in 
colloidal literature that colloids in general and proteins in particu- 
lar cannot form true aqueous solutions, 7.¢e., solutions in which the 
ultimate unit is a protein ion or a protein molecule. Instead, it 
was assumed that proteins form only suspensions in which the 
ultimate unit is an aggregate of molecules or ions, a so-called 
micelle. The forces responsible for true solutions are entirely 
different from the forces responsible for the stability of suspensions. 
While the stability of the isolated molecules or ions in true 
solutions is determined by the strong forces of attraction between 
molecules or ions of solute and water, the stability of solid aggre- 
gates (micelles) in water is determined by the weak forces of 
repulsion due to the electrical double layer between each particle 
and water. When the potential difference (P.p.) of this electrical 
double layer falls below a critical value (which is about 16 
millivolts for collodion particles in water)!” the particles will 
coalesce upon colliding, and settle rapidly. Low concentrations 
of neutral salts suffice to bring the p.p. between the particles 
and water below the critical limit, causing flocculation. Schulze,'® 
Picton and Linder,!® and Hardy?® have shown that the flocculat- 
ing effect of a salt increases rapidly with the increasing valency 

17 Lorn, J.: J. Gen. Physiol., 5 (1922-23), 109. 

18 Scuuuze, H.: J. prakt. Chem., 25 (1882), 431; 27 (1883), 320; 32 (1884), 
390. a 
19 Picton, H. and Linper, 8. E.: J. Chem. Soc., 61, 67, 71, 87. 

20 Harpy, W. B.:. Proc. Roy. Soc., 66 (1900), 110; 79 (1907), 413; J. 


Physiol., 29 (1903), 29; 33 (1905-06), 251; Woop, T. B. and Harpy, W. B.: 
Proc. Roy. Soc., 81 (1909), 38. 


36 COLLOIDAL BEHAVIOR 


of that ion of the salt which is charged oppositely to the 
particles moving in an electrical field. It requires generally, 
however, high concentrations of salts to cause precipitation of 
molecules and ions from true solution and the precipitating ion 
of the salt has not necessarily a charge opposite to that of the 
suspended particle. When this criterion is applied to solutions 
of genuine proteins, ¢.g., crystalline egg albumin or gelatin, it 
is found that they form true solutions, since enormous con- 
centrations of salts are required to precipitate these proteins 
from their solutions and since, moreover, the sign of charge 
of the active ion of the salt is not opposite to that of the 
protein. Sulfates precipitate solutions of gelatin better than 
chlorides, no matter whether the protein is at the isoelectric 
point or on the acid side of the isoelectric point or on the alkaline 
side (see Table III).24_ Such results are incompatible with the 
idea that the forces which keep protein in solution are the weak 
repulsive forces due to electrical double layers. 


Taste I1].—Mintmat Monar CoNnceNnTRATIONS REQUIRED TO PRE- 
CIPITATE 0.8 Per Crent SOLUTIONS OF GELATIN 





| Approximate molecular concentration of salt 


pH of gelatin solution required for precipitation 

















(NH,)280, | NasSO. | MgSO, | KCl | MgCl, 
4.7 (isoelectric gelatin). . 1546 M 68M 1¢%mM|>3m| >38™M 
3.8 (gelatin chloride)..... 1346 M 5g M 14 M 3M] >3™M 
6.4 to 7.0 (Na gelatinate) . 16/6 M 14M %umM |>3Mi| >3™M 


The only alternative is that these forces are the strong forces 
responsible for true solubility. The direct quantitative proof that 
proteins possess true solubility like any crystalloid was furnished 
by Cohn and Hendry” in an investigation of the relation between 
the solubility of casein and its capacity to combine with a base. 
This investigation was based on the principle of the constancy of 
the solubility product. It was found by these authors that 


21 Lorn, J.: “Proteins,” p. 245. 
22 Coun, E. J. and Henpry, J. L.: J. Gen. Physiol., 5 (1922-23), 521, 


BEHAVIOR OF PROTEINS Oo” 


casein forms a well-defined soluble disodium compound and that 
solubility was completely determined by (1) the solubility of the 
casein molecule, and (2) the concentration of the disodium casein 
compound. From the study of systems containing the protein 
and very small amounts of sodium hydroxide it was possible to 
determine the solubility of the casein molecule and also the degree 
to which it dissociated as a divalent acid and combined with base. 
Solubility in such systems increased in direct proportion to the 
amount of sodium hydroxide they contained. The concentration 
of the soluble casein compound varied inversely as the square root 
of the hydrogen ion concentration, directly as the solubility of the 
casein molecule, and as the constants Ka; and Kae defining its 
acid dissociation. ‘These investigations leave no doubt that the 
solubility of casein is adequately characterized as a true crystal- 
loidal solubility. There is also little doubt that this result can be 
applied to all genuine proteins. 

Two apparent difficulties have to be removed. Hardy, who 
had discovered the existence of the isoelectric point of protein 
particles in his famous experiments on the migration of particles 
of denatured egg albumin in an electrical field, had also noticed 
that the stability of suspensions of boiled white of egg was a 
minimum at the isoelectric point, and he ascribed this correctly 
to the fact that the cataphoretic p.p. between particles and 
water is a minimum at this point.23 This explanation cannot be 
applied, however, to aqueous solutions of genuine proteins, such 
as crystalline egg albumin, gelatin,etc. The solubility of genuine 
proteins in water is also a minimum at the isoelectric point and 
increases as a rule when alkali or acid is added, but for a different 
reason; namely, because protein salts and amphoteric electrolytes 
in general are more soluble in water than the non-ionized mole- 
cules. Since the ionization of proteins is a minimum at the 
isoelectric point, their solubility must also be a minimum at this 
point. This was pointed out already by Michaelis,?* and 
Michaelis and Davidsohn®> have shown that this is also true 
for amino acids, which are true crystalloids. 


23 Harpy, W. B.: Loc. cit. 

24 MicnHaELis, L.: “Die Wasserstoffonenkonzentration,” Berlin, 1914, 
p. 44. 

2 Micuag.is, L. and Davipsoun, H.: Biochem. Z., 30 (1910), 143. 


38 COLLOIDAL BEHAVIOR 


The second apparent difficulty lies in the fact that certain 
proteins, e.g., gelatin, form micelles on standing. When a 
solution of gelatin is left standing, it will set to a gel, if the 
concentration of gelatin is not too low, and the formation of a 
continuous gel is naturally preceded by the formation of smaller 
ageregates. But the formation of gels does not contradict the 
fact that the forces which keep gelatin in solution are those forces 
of attraction between the molecules or ions of gelatin and water 
which determine the true solubility of crystalloids like amino 
acids or any other substance, and which are designated by 
Langmuir as forces of secondary valency. On the basis of the 
well-known ideas developed by Langmuir and by Harkins it is 
necessary to distinguish in the case of large molecules between the 
relative affinity of each group of the molecule for water and for 
each other. Thus hydrocarbon groups are attracted more 
powerfully to each other than by water, and groups like NHz or 
NH# or COOH are attracted more strongly by water than by 
each other. Gelatin molecules or ions are dragged into the water 
by their NH, or NH3 and COOH or COO groups, but they are 
attracted to each other by their hydrocarbon groups. When 
two molecules of gelatin happen to come in contact with two 
hydrocarbon groups, they may remain attached to each other 
without any weakening of the attractive force between their N He 
or COOH groups and water, In a gel of gelatin the average 
distance between gelatin molecules is the same as in a gelatin 
solution; what is changed is only the orientation of the gelatin 
molecules towards each other.*® 

That the colloid chemists overlooked the fact that proteins 
possess true solubility is again the consequence of their failure to 
measure properly the hydrogen ion concentration of their solu- 
tions. Without such measurements it is, of course, impossible 
to prove the validity of the principle of the solubility product 
for proteins, or to prove that the active ion in salting out of pro- 
teins may have the same sign of charge as the protein ion. 
It is, therefore, no mere accident that all those authors who have 
measured the hydrogen ion concentrations of protein solutions, 
such as Michaelis and his follow workers, Sérensen,?’ E. J. Cohn, 


26 Lons, J.: ‘Proteins,’ p. 243. 
27 SORENSEN, S. P. L.: Compt.-rend. trav. lab. Carlsberg, 12 (1915-17), 6. 


BEHAVIOR OF PROTEINS 39 


and the writer, have reached the conclusion that proteins form 
true solutions. 


THE COLLOIDAL BEHAVIOR OF PROTEINS 


Membrane, Equilibria and Their Equations.—If proteins 
behave like crystalloids, chemically and in regard to solubility, 
the question may be asked: Why are proteins termed colloids? 
The answer is that proteins show colloidal behavior only in regard 
to the influence of electrolytes on four well-defined properties, 
namely, membrane potentials, osmotic pressure, swelling, and 
that form of viscosity which is due to the swelling of submicro- 
scopic particles. This influence of electrolytes is similar on all 
four properties and may be summarized in the following way: 

1. The addition of little acid or alkali to isoelectric protein 
increases at first the value of these four properties until a maxi- 
mum is reached, after which the addition of more acid or alkali 
diminishes the value of these properties again. 

2. This influence of acids and alkalies depends only on the 
valency, and not upon the chemical nature of the anion of the acid 
or the cation of the alkali. It is, e.g., the same for all acids the 
anions of which are monovalent, provided the effects of different 
acids on the four properties mentioned are compared at the same 
pH of the protein solution or protein gel. 

3. When the anion of the acid or the cation of the alkali is 
bivalent (e.g., S04, Mg, Ca, Ba, etc.), the membrane potentials, 
osmotic pressure, viscosity, and swelling of the protein are 
considerably less than when the ion is monovalent (e.g., Cl, 
Br, NOs, H2PO,, HC,.O,, ia: Na, Le NH,, etc.). 

4. The addition of a neutral salt to a protein solution or protein 
gel (not at the isoelectric point) depresses the value of the four 
properties and this depressing effect increases with the valency 
of that ion of the salt which has the opposite sign of charge to that 
of the protein ion. The chemical nature of the active ion of the 
salt has no direct influence on these four properties but may affect 
some of them, e.g., swelling or viscosity, indirectly by influencing 
the cohesion of a gel or its solubility. 

No such influence of electrolytes is observed on amino acids or 
on other typical crystalloids, and the question arises: Which pecu- 
liarity of the proteins gives rise to this specific influence of electro- 


40 COLLOIDAL BEHAVIOR 


lytes on the four properties mentioned? The answer is that the 
peculiarity in question is the large protein ion which is prevented 
from diffusing through many membranes or through gels easily 
permeable to the smaller ions of the ordinary crystalloidal elec- 
trolytes. This selective diffusion is the basis of the method of 
dialysis as well as of a peculiar equilibrium condition whereby 
the concentration of the diffusible ions is, at equilibrium, not the 
same inside a protein solution and in an outside aqueous solution, 
free from protein, separated by a dialyzing membrane. ‘This 
unequal distribution of the diffusible ions on the opposite sides 
of a dialyzing membrane separating a protein solution and an 
aqueous solution free from protein, when equilibrium is estab- 
lished between the two solutions, is the sole cause of the pe- 
culiar influence of electrolytes on the four properties of proteins, 
and, hence, the sole cause of the colloidal behavior of proteins. 
The theory of such membrane equilibria has been developed 
by Donnan. 

Suppose a collodion bag of a volume of about 50 ce. is filled with 
a solution of gelatin chloride of pH 3.0, containing 1 g. of origin- 
ally isoelectric gelatin in 100 cc. of solution. The bagisclosed with 
a rubber stopper perforated by a glass tube serving as a manom- 
eter. The collodion bag is submerged in 350 cc. of a solution 
of HCl originally also of pH 3.0. Water will diffuse into the gela- 
tin solution, the level of water rising in the manometer until 
finally a definite level is reached which will remain constant. 
This level is the hydrostatic pressure at which the system is in 
osmotic equilibrium. The equilibrium is established at 24°C. 
after about 6 hours, but it is better to wait 18 hours before 
measurements are taken. It is found that when equilibrium is 
established, the concentration of the H and Cl ions is not the 
same inside the gelatin solution and in the outside aqueous 
solution. ?8 

The gelatin chloride solution inside the bag is dissociated into 
gelatin ions and Cl ions. The molar concentration of the 
latter may be designated asz. In addition, there exists free HCl 
inside the gelatin chloride solution due to hydrolytic dissociation 
of the gelatin salt as shown by the titration and combination 
curves. Let y be the molar concentration of the H and of the Cl 

28 Lous, J.: ‘‘ Proteins,” p. 169; Science, 56 (1922), 731. 


BEHAVIOR OF PROTEINS 4] 


ions of the free HCl inside the gelatin solution at equilibrium. 
Then the total molar concentration of H ions inside the protein 
solution at equilibrium is y and that of the Cl ions y + z. 

Let x be the molar concentration of the H and Cl ions in the 
outside solution at osmotic equilibrium. Since it can be shown 
experimentally that the collodion membrane is impermeable 
to the solution of most if not all proteins, but perfectly permeable 
to H and Cl ions, at osmotic equilibrium the distribution of H 
and Cl ions on the opposite sides of the membrane must be 
determined by Donnan’s equation for membrane equilibria, 
according to which the products of the molar concentrations of 
each pair of oppositely charged ions must be equal on the opposite 
sides of the membrane, 2.e., in the case of gelatin chloride solutions, 


x? = yy + 2) Ok 


This equation is the same for all acids with monovalent anion. 
When the anion is bivalent, the equilibrium equation is one of 
the third degree; namely, 


oh= yy +2) (a) 


Bi Xs 5 : ‘ ‘ e 
where 5 is the molar concentration of the anion in combination 


with the gelatin. Only the valency but not the chemical nature 
of the anion of the acid enters, therefore, into the equations for 
the Donnan equilibrium. 

Membrane Potentials.—To 1 g. dry weight of isoelectric gelatin 
were added different numbers of cubic centimeters of 0.1 N solu- 
tions of various acids, HCl, HBr, HI, HNOs, acetic acid, etc., 
and the total volume was brought to 100 cc. by the addition of 


27 Donnan, F. G.: Z. Elektrochem., 17 (1911), 572: Lewis, W. C. McC.: 
“A System of Physical Chemistry,” vol. 2, p. 399, London, 1920; Proctsr, 
H. R. and Witson, J. A.: J. Chem. Soc., 109 (1916), 307; Lous, J.: ‘‘Pro- 
teins”; J. Gen. Physiol., 3 (1920-21), 667, 691, 827; 4 (1921-22), 73, 97; 
Boausr, R. H.: ‘The Chemistry and Technology of Gelatin and Glue,” 
New York, 1922, p. 128; Witson, J. A.: ‘‘The Chemistry of Leather Manu- 
facture,’’ New York, 1923, p. 94. 

30Lons, J.: ‘‘Proteins,’”’ p. 120; J. Gen. Physiol., 3 (1920-21), 667; 4 
(1921-22), 351, 769; Lorn, J, and Kunirz, M.: J. Gen. Physiol., 5 (1922- 
23), 665. - 


42 COLLOIDAL BEHAVIOR 


distilled water. Collodion bags of about 50-cc. content were 
filled with these protein solutions and each bag was closed with 
a rubber stopper perforated by a glass tube serving as a manom- 
eter. Each bag was put into 350 cc. of water free from protein 
but containing some of the same acid as that added inside to the 
gelatin. This wasdone to hasten the establishment of equilibrium. 
After 18 hours the height of the column of H,O in the glass tube 
was measured (giving the osmotic pressure of the protein solution), 
and the p.p. was then determined between the protein solution 
and the outside aqueous solution by means of a Compton electrom- 
eter with two saturated KCl-calomel electrodes. The e.m.f. 
of the following cell was, therefore, measured: 

















saturated gelatin | outside saturated 
Hemerte el KCl acid collodion || aqueous KCl HgCl | Hg 
solution solution || membrane || solution solution 
| = 














This e.m.f. will be called the membrane potential. Figure 4 
gives the results. The abscisse are the pH of the protein solu- 
tions at equilibrium (determined with the hydrogen electrode) 
‘and the ordinates are the p.p. measured with the electrometer. 
First it is noticeable that the membrane potentials are a minimum 
at the isoelectric point, that they rise with diminishing pH (z.e., 
increasing hydrogen ion concentration) until a maximal P.D, is 
reached at about pH 4.0, and that with a further diminution of 
the pH the potentials fall again.*! 

That the membrane potentials are due to the Donnan equilib- 
rium follows from the following facts: 

1. The values for the influence of all the acids with monovalent 
anion, HCl, HBr, HI, HNOs, acetic, propionic, and lactic acids, 
on the membrane potential between gelatin solution and outside 
aqueous solution lie on one curve (Fig. 4). The values repre- 
senting the influence of the two strong dibasic acids, H2SO, 
and sulfosalicylic acid, on membrane potentials lie also on one 
curve (Fig. 4), but this curve is lower than that for the monobasic 
acids.22. The chemical nature of the anion plays no rdle, as 
Donnan’s equation demands, since the membrane equilibria 

31 Logs, J.: ‘ Proteins,” p. 122; Lous, J. and Kunitz, M.: J. Gen. Physiol., 


5 (1922-23), 671. . 
32 Logs, J. and Kunitz, M.: J. Gen. Physiol., 5 (1922-23), 671. 


BEHAVIOR OF PROTEINS 43 


are purely electrostatic equilibria depending only on the number 
of charges but not on the chemical nature of the ions. No other 
physical properties, except those due to the Donnan equilibrium, 








Millivolts 
ro 
ae 













esas 
: Pama 
Bec 





16 18 20 22 04 26 28 50 ac 34 36 38 40 42 is 46 48 5.0 
pH 
Fic. 4.—Proof that only the valency of the anion of an acid influences the 
membrane potentials of gelatin solutions. The ordinates are the membrane 
potentials in millivolts; the abscisse the pH of gelatin solutions. The mem- 
brane potentials of the seven monobasic acids are practically identical and so 
are the membrane potentials of the two strong dibasic acids. 


show this peculiarity, that only the valency but not the chemical 
nature of the ion has any effect on the colloidal properties. 


44 COLLOIDAL BEHAVIOR 


| 


2. Donnan’s equilibrium equation for monobasic acids can be 
written in the form 


+z 


x 


SIs 


Donnan has shown that there must exist between the inside and 
outside solution a p.p. as follows: 
RT 


MEM cots oe Sas 
Dae ah Be 


where z is the molar concentration of hydrogen ions outside, and 
y the molar concentration of hydrogen ions inside. Since pH 
outside is —log x and pH inside —log y, the membrane potential 
measured with the indifferent calomel electrodes should be equal 
to the hydrogen electrode potential between the gelatin solution 
and the outside aqueous solution, if Donnan’s membrane equilib- 
ria are the cause of the membrane potentials. This was found 
to be true within the limits of the accuracy of the measurements 
(about 2 millivolts). By hydrogen electrode potentials is under- 
stood the value 59 X (pH inside minus pH outside) millivolts, 
where each pH is measured between a calomel-saturated KCI 
electrode and a hydrogenelectrode. Whatwas actually measured 
was the difference in the e.m.f. of the following two cells: 


(a) 








inside | 
H, gelatin chloride | saturated KCl | HgCl_ | Hg 
solution 
(6) 
He purge . saturated KCl | HgCl | Hg 
aqueous solution 








In Fig. 5 are given the hydrogen electrode potentials of the same 
acids as in Fig. 4. The curves for the hydrogen electrode p.p. 
in Fig. 5 and the curves for the membrane p.p. in Fig. 4 are 
identical. The hydrogen electrode p.p.’s for all the monobasic 
acids are, therefore, the same within the limits of accuracy of 
measurements and the pP.p.’s for the two strong dibasic acids 
are also the same in both Figs, 4 and 5. 





BEHAVIOR OF PROTEINS Ad 


3. Loeb had shown furthermore that if Donnan’s membrane 


equilibrium is responsible for the effect of acids on the membrane 


Millivolts 


_ potentials of protein solutions, the effects of monobasic acids 


Bi 
sof 
s[— 
aS 
34 
32 
30 
28 
26 
24 
22 
20 







oe 
ee 
SEUaSEEBELGcoaaS 
Soe ae 


.. SSR aaa ee 
em se a 
_) SS Se AS ee 
JES ane again 
eae 
fs tunes i: acla cas 
Beene eet 
Ae ae 
Open 
__ @ (2S aes eee 

ooo eee 

eee oe 
a [ote ST SD We 
ise ceuct 26) eB (40; 52 ek 36 38 40 42 44 46 48 50 
P 


















Fig. 5.—Proof that the influence of acids on the hydrogen electrode potentials 


of gelatin solutions is identical with that on the membrane potentials as shown 
in Fig. 4. 


should be exactly 50 per cent higher than those for dibasic acids 
at the same pH on the basis of the following consideration. 


46 COLLOIDAL BEHAVIOR 
From equation (1) it follows that in the case of monobasic 
acids « = \/y(y 4+-2). Substituting this value for z in the term 


fe 
—, we get 
y g 





Hence, the membrane potential of a protein solution should be at 
24° for monobasic acids 


PD. = ee log G ~{- A millivolts 
From equation (2) it follows that in the case of dibasic acids 
V yy + 2) 
Substituting this value in : we get 
£ VEU tS) _ wre) _ ore eee 
y y y° y y 


The p.p. is, therefore, in the case of a dibasic acid 


xv 











58 Z ane 
LO ae log G + a) millivolts 


Hence, at the same pH of the gelatin solution the ratio of the 
P.D. of gelatin sulfate over that of gelatin chloride must be as 
2:3, or 0.66.33 

A comparison of the effects of sulfosalicylic acid with those for 
HCl and the other monobasic acids at the same pH in Fig. 5 
shows that this is correct within the limits of experimental ) 
accuracy (Table IV). | 

The values for sulfosalicylic acid were used in preference to the | 
values for sulfuric acid, for the reason that a repetition of the . 
experiment with sulfuric acid showed that the values for sulfo- 
salicylic and sulfuric acids are, in reality, identical, and that the 
values for sulfuric acid given in Fig. 4 are a little too low. 

Most weak dibasic and tribasiec acids dissociate as monobasic 
acids below a certain pH. H;POy, dissociates as monobasic acid 


°? Lorn, J.: “Proteins,” p. 132. 
** Logs, J. and Kunrrz, M.: J. Gen. Physiol., 5 (1922-23), 675. 


BEHAVIOR OF PROTEINS 47 


TasLeE 1V.—MeEMBRANE POTENTIALS FOR Drpasic AND MONOBASIC 














AcIps 
pH Dibasic acids, | Monobasic acids, mens dibasic 
millivolts millivolts monobasic 
2.4 T.6 11.4 0.67 
2.6 9.6 14.8 0.65 
2.8 11.6 18.0 0.64 
3.0 13.6 Zio 0.65 
So LD 24.8 0.64 
3.4 18.0 28.0 0.62 
3.6 19.8 BLU 0.64 
5 ie! POY) 34.2 0.62 
4.0 21.6 ey, 0.61 
4.2 20.8 34.8 0.60 
4.4 19.2 31.0 0.62 








below pH 4.7 and it had been shown that in this range of pH the 
influence of H;PO, on membrane potentials (as well as osmotic 
pressure, swelling, and viscosity) is identical with that of HCI or 
any other monobasic acid if compared for the same pH of the 
protein solution or gel. Oxalic acid dissociates as a monobasic 
acid below pH 3.0 and it had been shown that for pH of 3.0 or less 
the influence of the oxalic acid on the properties mentioned is 
like that of HCl. Above pH 3.0 the second H ion of the oxalic 
acid begins to dissociate and the relative number of dibasic anion 
increases with a further increase of pH and, hence, the depressing 
effect of the dibasic anion is felt more and more the higher 
the pH.*° 

After these remarks, the effect of succinic, citric, and tartaric 
acids on the membrane potentials as plotted in Fig. 6 is easily 
understood. All three acids act like HCl below pH 3.0, 2.¢., the 
curve representing the influence of these three acids on the mem- 
brane potential coincides with that for HCI, but not with that for 
H.SO., which means that all these acids dissociate for pH < 3.0 
as monobasic acids, and, furthermore, it is clear that these weak 
dibasic acids behave as the valency rule demands, 


3 Lorn, J.: ‘‘Proteins,” pp. 122, 127. 


48 COLLOIDAL BEHAVIOR 


Above pH 3.0 the curves for succinic, citric, and tartaric acids 
are lower than the curve for HCl but considerably higher than 
that for H250.4, which means that at a pH > 3.0 the second H ion 
of the weak dibasic and tribasic acids begins to be split off, 






















meee 
eaneeer 
ou ARIAT of 
Seats) la 
; a Be EECCA 
/| 
we 
A 





Re ‘j 
ity BERR 
a ae 
Seeapeueneseaeeee 


lo 0 22 24 26 28 30 32 34 So 38 40 42 44 AG 48 50 

pH 

Fic. 6.—Influence of weak dibasic and tribasic acids on the membrane potentials 
of gelatin solutions. 





and the more, the stronger the acid. Thus, in the case of the 
weak succinic acid only a very small percentage of molecules 
dissociates as dibasic acid and the same may be said for citric 
acid, while a greater percentage of tartaric acid molecules 
dissociates as dibasic acid between pH 3.0 and 4.7. These 


ee eS ee ee eT ee he 


= Oe 


BEHAVIOR OF PROTEINS AQ 


experiments might almost be used as a criterion for the mode of 
dissociation of weak dibasic and tribasic acids. *° 


58 
4. The term P.D. = 9 log G1 + ) gives also an explanation 


of why the addition of little acid to isoelectric gelatin increases the 
membrane potentials until a maximum is reached, after which the 
addition of more acid diminishes the p.p. again. The addition 
of acid to originally isoelectric protein increases the value of 2, 
z.e., the concentration of ionized protein acid salt, as well as the 
value of y, z.e., the concentration of anion; but at first z increases 
more rapidly than y, until a certain percentage of protein is 
ionized when, with the addition of more acid, the value of z 
increases less rapidly than that of y.*” 

5. It is also obvious from the above term for the p.p. why a 
salt can only depress but cannot raise the p.p. The addition of a 
salt cannot increase the value of z, 7.e., the concentration of 
ionized protein, while with the increase in the concentration of 
the salt the value of y, 2.e., the anion of the protein salt, will 
increase. *8 

6. The membrane potentials as measured by the two indifferent 
calomel electrodes must also be equal to the chlorine ion potentials 
if the membrane equilibria are the cause of the p.p., and the 
writer’s measurements have shown this to be true within the 
limits of the accuracy of the measurements. *? 

7. The value of the membrane potentials must increase with 
the concentration of the protein in solution, since this increases 
the value of z in equations (1) and (2), and this was also found 
to be correct. *° 

These facts leave no doubt that the influence of electrolytes on 
the membrane potentials between protein solutions and outside 
aqueous solutions can be explained quantitatively from Donnan’s 
theory of membrane equilibria. 

Osmotic Pressure of Protein Solutions.—The same experi- 
ments which were used for the measurement of membrane poten- 


36 Lorns, J. and Kunitz, M.: J. Gen. Physiol., 5 (1922-23), 677. 
7 Loxs, J.: “Proteins,” p. 131. 

88 Tbid.: p. 143. 

M ibid. De 135) 

40 Tbid.: p. 145, 


m= 


50 COLLOIDAL BEHAVIOR 


tials were also used for measuring the osmotic pressure of gelatin 
solutions containing | g. dry weight of originally isoelectric gela- 
tin in 100 cc. of water made up with various acids. The results 
are contained in Fig. 7. The ordinates are the observed osmotic 


ate alae lead eet: 
[EE ESE Acetic acid 
PRaGMeoe.ce 
















, Y 
oH Aes 
oe OM 
(RRR ant 
Lael aee 

ol | bl 
PARE e 
et) topes 
Eco 
pails | <2] (S/ S| a aa 

Ln aA 
eae ae 

ane 

100 .} 

ST | ee Bm 
S SRRRRRRRESEEEE Y 





rm 


22 24 26 28 30 32 34 36 38° 40 42 44 46 48 
pH 

Fic. 7.—Proof of valency rule for the influence of acids on the osmotic pressure 

of gelatin solutions. The influence of seven monobasic acids on the osmotic 


pressure of gelatin solutions is the same and about twice as high as that of the 
two dibasic acids. 


pressures in terms of millimeters of a column of water, and the 
abscissee are the pH of the gelatin solution at equilibrium. 
It is obvious that the osmotic pressure of the gelatin solution is a 
minimum at the isoelectric point, that it rises upon the addition 


BEHAVIOR OF PROTEINS 51 


of acid until a maximum is reached at pH 3.3, and that, upon the 
further addition of acid, the osmotic pressure diminishes again. 
It is also noticeable that all the monobasic acids influence the 
osmotic pressure in exactly the same way; and the values for 


SSeS a 

| See aN TS 

Planetree! YAP ONY ich] tt 
re 


415 
450 
























AY {a 
ri 
EECCA 
|) SRS 
eee eee 
_ | SSSR RRR Nee 
OSS SEN 
a 
mammeere | LT AAA 
AC RRR. ae 
eet ff fA 
Bee i TT 
HE jp 


OF 18 20 22 2h 26 28 30 32 34 36 38 40 42 44 46 48 50 
pH 
Fig. 8.—Influence of weak dibasic and tribasic acids on the osmotic pressure of 
gelatin solutions. 





200 


Osmotic pressure mm. He0 


HCl, HBr, HI, HNOs, acetic, propionic, or lactic acids lie practi- 
cally all on one curve. The osinotic pressure curves for the two 
strong dibasic acids, H.SO, and sulfosalicylic acid, also fall on 
one curve, which is, however, entirely different, being about half 
as high as the curve for the monobasic acids for the same pH.*! 

41 Tbid.: p. 169; Lons, J. and Kunitz, M.: J. Gen. Physiol., 5 (1922-23) 
665. 


52 COLLOIDAL BEHAVIOR 


It had been shown in preceding papers and in a book that the 
curve representing the influence of H3;PO, on the osmotic pressure 
of a gelatin solution is identical with the curve representing the 
influence of HCl, if both are plotted over the pH of the gelatin 
solution as abscisse; and that the curve for oxalic acid is also 
identical with the curve for HCl and H3POs, for pH 3.0 or below, 
while for pH above 3.0 the influence of the bivalent oxalate anion 
becomes noticeable in the fact that the osmotic pressure for oxalic 
acid is, in that range of pH, lower than for HCI.*2 

Figure 8 represents the influence of succinic, citric, and tartaric 
acids on the osmotic pressure of a solution containing 1 g. dry 
weight of originally isoelectric gelatin in 100 ce. solution. As was 
to be expected, the descending branches of the curves for these 
acids are identical with the corresponding part of the curve for 
HCl for pH below 3.0, while above pH 3.0 the curves for the three 
weak dibasic or tribasic acids are slightly lower in the order of their 
relative strength as discussed in connection with the membrane 
potentials. *% 

It would be possible to use the influence of dibasic or tribasic 
acids on the osmotic pressure of pipet solutions to determine 
their relative strengths. 

Now the question arises: What causes this influence of acids on 
osmotic pressure? First, the fact shown in Figs. 7 and 8, that 
only the valency but not the chemical nature of the anion of the 
acid influences the osmotic pressure, is to be expected if this 
influence of the acid is due to the Donnan equilibrium. An 
equally important task is, however, to explain why the addition 
of little acid raises and why the addition of more acid depresses 
again the osmotic pressure. The colloid chemists would have 
taken it for granted that such curves were due to an influence of the 
acids on the state of dispersion or on some other real or imaginary 
colloidal property of proteins. Before we have a right to indulge 
in such speculations, we must realize that these curves of observed 
osmotic pressure are not exclusively the expression of the osmotic 
pressure due to the protein particles, protein molecules, and pro- 
tein ions alone, but are also the result of the demonstrable unequal 
concentrations of the crystalloidal ions on the opposite sides of 


42 Lors, J.: ‘‘Proteins,” p. 174. 
43 Lous, J. and Kunitz, M.: Loc. cit. 


BEHAVIOR OF PROTEINS 53 


the membrane, caused by the establishment of a Donnan equilib- 
rium. In other words, the observed osmotic pressure of a protein 
solution needs a correction due to the Donnan equilibrium 
before we can begin to speculate on the cause of the influence of 
acid on these curves, and it is our purpose to calculate the value 
of this correction. 

We begin with the curve expressing the influence of HCI on the 
osmotic pressure of a 1 per cent solution of originally isoelectric 
gelatin and we consider the distribution of ions inside the protein 
solution and in the aqueous solution outside the collodion bag 
containing the protein solution at osmotic equilibrium. We also 
assume complete electrolytic dissociation of gelatin chloride as 
well as HCl. Let a be the molar concentration of the protein 
molecules and ions, let z be the molar concentration of the Cl ions 
in combination with the ionized protein, let y be the molar con- 
centration of the hydrogen ions of the free HCI inside the protein 
solution; the molar concentration of the Cl ions of this HCl is 
also y. In that case the osmotic pressure of the protein solution 
is determined by the molar concentration 


QZ ie 


From this must be deducted the osmotic pressure of the HCl of 
the outside aqueous solution. If x is the molar concentration 
of the H ions of the outside solution, it is also the molar 
concentration of the Cl ions. Hence the observed osmotic 
pressure of a protein solution is determined by the following 
molar concentration 


Cir 2 20 


Figure 7 shows how this value varies with the pH of the protein 
solution (7.e., with y). In order to arrive at a theory concerning 
the influence of HCl on the osmotic pressure of protein solutions, 
it is necessary to calculate the osmotic pressure due to the value 
of 2y + 2-— 2x and to deduct it from the observed osmotic 
pressure of the protein solution. The osmotic pressure deter- 
mined by the value 2y + 2 — 2% we will call the ‘“‘ Donnan cor- 
rection.”’ In this term, y and « can be calculated from the 
measurements of the pH, pH inside being — log y and pH outside 


o4 COLLOIDAL BEHAVIOR 


being — log x. 2 can be calculated from x and y with the aid of 
the Donnan equation (1) 


ee Pye) 













lo t8 20 220 24)° 726 28 Et be 54 56 38 40 42 44 46 
pH 
Fig. 9.—Showing agreement and minor discrepancies between the curves 


of observed and calculated osmotic pressures of 1 per cent gelatin chloride 
solutions. 


0 


since we now know through the experiments on membrane poten- 
tials that x and y are determined by the Donnan equilibrium. 
If the value of 2y + 2 — 2a is calculated for different pH of a 
gelatin chloride solution (of the same concentration of originally 
isoelectric gelatin, which in this case was 1 per cent); and if from 
this value is calculated the osmotic pressure due to this excess 


BEHAVIOR OF PROTEINS 55 


of the molar concentration of crystalloidal ions inside the protein 
solution over that outside, on the basis of van’t Hoff’s theory of 
osmotic pressure, it is found that the curve for the Donnan cor- 
rection is almost, but not quite, identical with the curve for the 
observed osmotic pressure (Fig. 9). In other words, it turns out 
that the increase in osmotic pressure of a 1 per cent solution of 
originally isoelectric gelatin upon the addition of little acid 
until a maximum is reached, and the diminution of osmotic pres- 
sure upon the addition of further acid, are not due to any varia- 
tion in the state of dispersion of the protein, or any other real 
or imaginary ‘‘colloidal’”’ property of the protein, but purely to 
the fact that protein ions cannot diffuse through the collodion 
membrane, which is easily permeable to crystalloidal ions. As 
a consequence, the molar concentration of the crystalloidal ions 
must always be greater inside the protein solution than outside. 
What varies with the pH of the gelatin solution is the value of 
2y +2-— 2x. This follows from the Donnan equation (1), 
according to which 


t= Vy? + yz or 22 = W/4y? + 4yz 
while 


2y +z = W/4y? + 4yz 4+ 2? 
Now, it is obvious that 
v/4y? + 4yz + 22 > / Ay? + 4yz 

7.e., the concentration of the crystalloidal ions inside the protein 
solution 2y + z is always greater than the concentration of the 
crystalloidal ions 2x outside, when z is not 0 or ©. 

If we substitute for the term 2y + 2 — 2x of the Donnan 
correction the identical term 


V 4y? + 4yz2 + 2? — V/4y? + 4yz 
we can visualize why the osmotic pressure is a minimum at the 
isoelectric point, why it increases with the addition of little acid, 
reaching a maximum, and why it diminishes again with the 
addition of more acid.*4 

At the isoelectric point no protein is ionized and, z being zero, 

the whole term | 

V Ay? + 4yz2 + 2? — / Ay? + 4yz 


44 Longs, J.: Science, 56 (1922), 731. 


56 COLLOIDAL BEHAVIOR 


becomes zero. Hence, at the isoelectric point the observed 
osmotic pressure is purely that due to the protein, which is very 
low on account of the high molecular weight of gelatin. 

When little acid, e.g., HCl, is added to the solution of isoelectric 
gelatin, gelatin chloride is formed and some free acid remains, 
due to hydrolytic dissociation. Hence both z (the concentration 
of Cl ions in combination with protein) and y (the Cl ions of the 
free HCl existing through hydrolysis) increase, but z increases at 
first more rapidly than y, and, hence, the excess of concentration 
of ions inside over that of ions outside increases until the greater 
part of protein is transformed into protein chloride, when the 
excess of crystalloidal ions inside over those outside reaches a 
maximum. From then on Zz increases comparatively little, 
while y increases considerably with further addition of acid, so 
that z becomes negligible in comparison with y. This explains 
why the Donnan correction becomes zero again when enough 
acid is added, and why the observed osmotic pressure becomes as 
low again as at the isoelectric point. 

In the same way it can be shown why the addition of salt has 
only a depressing effect on the osmotic pressure. Let us assume 
that there is inside the bag a gelatin chloride solution of pH 3.0 
to which NaCl is added. z (the concentration of Cl ions in com- 
bination with the gelatin) will not increase with the addition of 
salt, while y (the concentration of the Cl ions not in combination 
with gelatin) will increase. Hence, with the increase in the con- 
centration of the salt the value of 

a/4y? + AZ /4y? + 4yz 
will become smaller, finally approaching zero. 

When another salt than a chloride, e.g., NaNOs, is aed to a 
solution of gelatin chloride, we may assume that the gelatin in 
solution becomes gelatin nitrate. 

Figure 9 gives a comparison of the curves for the observed 
osmotic pressure and for the osmotic pressure calculated from the 
Donnan correction. Both curves rise in a parallel way from the 
isoelectric point, reaching a maximum which is 450-mm. water 
pressure in the case of the observed osmotic pressure and slightly 
lower in the case of the Donnan correction. The observed 
osmotic pressure should be higher than the osmotic pressure 


BEHAVIOR OF PROTEINS 57 


calculated from the Donnan correction by the osmotic pressure 
due to the protein molecules and ions. An almost constant 
difference exists in the two curves between pH 4.6 and 3.2, but 
disappears later, and this difference is in all probability the 
expression of the value of a, 7z.e., the osmotic pressure due to the 
protein itself. The disappearance of this difference at pH below 
3.2 is probably due to the fact that an error of one unit in the 
second decimal of the pH causes a considerable error in the cal- 
culations of z, which increases when the pH becomes smaller. 

Figure 9 shows that when we correct the observed osmotic 
pressure for the Donnan effect it follows that the influence of the 
pH of the acid on the osmotic pressure is entirely or practically 
entirely due to the excess of the concentration of crystalloidal ions 
inside the membrane over that outside and that this excess is 
caused by the Donnan equilibrium. The osmotic pressure of the 
protein itself is either not altered at all by the addition of acid or, 
if it is altered, the effect is too small to be noticeable. There is 
then nothing left for the ‘‘dispersion theory”’ or for any other of 
the colloidal speculations to explain. This conclusion was 
confirmed by experiments on crystalline egg albumin and casein 
by the writer* and on edestin by Hitchcock. *® 

We can, therefore, summarize these results by stating that the 
so-called colloidal behavior of protein solutions, asfar as membrane 
potentials and osmotic pressure are concerned, is merely the 
result of an equilibrium condition of classical chemistry, which 
results in an excess of the concentration of crystalloidal ions inside 
the protein solution over that of an outside aqueous solution, 
when the two solutions are separated by a membrane which is 
permeable to crystalloidal ions but impermeable to protein ions. 
‘The colloidal behavior of proteins depends, therefore, entirely 
on the relative non-diffusibility of protein ions through membranes 
which are easily permeable to crystalloidalions. Since the major- 
ity of membranes in plants and animals belong to this class, 
it can easily be surmised how great a role the proteins must 
play in the regulation of osmotic pressure in the body and the 
distribution of electrolytes between the body liquids and 
the cells. 


45 Lorn, J.: J. Am. Chem. Soc., 44 (1922), 1930. 
46 Arircucock, D. I.: J. Gen. Physiol., 4 (1921-22), 597. 


58 COLLOIDAL BEHAVIOR 


Swelling.—Procter and Wilson*’ have shown that the influence 
of HCl on the swelling of gelatin is a purely osmotic effect. 
The acid, combining with the gelatin, causes salt formation, 
the gelatin ions being prevented from diffusing by the cohesive 
forces between the gelatin ions or molecules of the gel. Since 
the gel is freely permeable to water and crystalloidal ions, such 
as H and Cl, the non-diffusibility of the gelatin ions causes the 
establishment of a Donnan equilibrium between gelatin and out- 
side solution, as a result of which the total molar concentration 
of all the diffusible crystalloidal ions is greater inside than outside 
the gel. This causes the influence of the acid on the swelling of 
gelatin and this influence is the same as that on osmotic pressure, 
for the reason that the influence of acid on swelling is also an 
osmotic pressure effect. The difference between the effect of 
acid on the osmotic pressure of gelatin solutions and on the 
swelling of gelatin gels is simply this—that in the former case the 
diffusion of the gelatin ions is blocked by the collodion membrane, 
and in the latter case by the cohesive forces between the gelatin 
molecules or gelatin ions of the gel. 

These cohesive forces are also the limiting force to the swelling 
of agel. Isoelectric gelatin absorbs a certain quantity of water, 
due to forces which have probably nothing to do with the Donnan 
equilibrium, since at the isoelectric point protein is only slightly 
ionized. The absorption of water by isoelectric gelatin is deter- 
mined by forces of attraction between certain groups of the gela- 
tin molecule and water, and is primarily, though perhaps not 
exclusively, a case of solid solution.*® The additional swelling 
caused by the addition of acid is, however, as Procter and Wilson 
have shown, an osmotic phenomenon due to the excess in the 
concentration of H and Cl ions inside over that outside. This 


causes the diffusion of water into the gel. The hydrostatic. 


pressure of the water will force the molecules of the gel apart and 
this will cause an increase in the forces of cohesion, which will 


47 Procrmr, H. R.: J. Chem. Soc., 105 (1914), 313; Procter, H. R. and 
Witson, J. A.: J. Chem. Soc., 109 (1916), 307; Wiztson, J. A. and WIxson, 
W. H.: J. Am. Chem. Soc., 40 (1918), 886; Wiuson, J. A.: J. Am. Leather 
Chem. Assoc., 12 (1917), 108; ‘““The Chemistry of Leather Manufacture,”’ 
New York, 1923. 

48 Lous, J.: ‘Proteins,’ p. 193. 


Se ee a 


BEHAVIOR OF PROTEINS 59 


oppose the further swelling. To give an idea of the difference 
between the swelling of isoelectric gelatin and that due to the 
influence of acid, it may be stated that while 1 g., dry weight, of 
powdered isoelectric gelatin absorbed about 7 g. of water, the 
same gelatin, when under the influence of an acid with monobasic 
anion, absorbed about 35 g. of water at pH 3.2 or 3.0 (of the gel), 
where the swelling is a maximum.*®? The forces of cohesion 
between the molecules or ions of the gel may be modified by the 
solute, e.g., the anion of the acid, and when this happens, the 
pure osmotic pressure effect, due to the Donnan equilibrium, may 
not be observed. ‘This was noticed in the effect of acids on the 
swelling of casein, where it was found that swelling occurs in HCl 
or HNOs, but not in trichloroacetic acid.°° These secondary 
effects of the anion of the acid or of the undissociated acid on the 
cohesion of the gel are slight and negligible in the case of a gel 
of gelatin, and, for this reason, the validity of the valency rule 
can easily be demonstrated for the influence of acids on the swell- 
ing of gelatin. 

The method of calculating the effect of HCl on the swelling of 
the gel from the Donnan equilibrium is similar to that for calcu- 
lating the osmotic :pressure, but is complicated by the necessity 
of introducing the cohesive forcesof the jelly. Since space forbids 
to go into the derivation of the equation of Procter and Wilson, 
the reader is referred to their original papers or to Wilson’s®! 
or the writer’s book.®? Chapters I and XXX of this treatise 
by Wilson and Procter respectively develop this point. Figures 
1 and 2 of Chapter I (pages 17—18) show the excellent agreement 
between Procter’s and Wilson’s values calculated on the basis 
of Donnan’s theory of membrane equilibria and the observed 
values for swelling. 

If the influence of acid on the swelling of gelatin is due to the 
Donnan equilibrium, the influence of different acids on swelling 
must depend solely on the valency but not the nature of the anion 


49 Lors, J. and Kunirz, M.: J. Gen. Physiol., 5 (1922-23), 665. 

50 Lorn, J. and Loss, R. F.: J. Gen. Physiol., 4 (1921-22), 487; Lozs, J.: 
“Proteins,” p. 193. 

51 Witson, J. A.: “The Chemistry of Leather Manufacture,’ New York, 
1923. 

52 Lorn, J.: ‘ Proteins,” p. 190. 


60 COLLOIDAL BEHAVIOR 


of the acid. Figure 10 gives the results with different acids. 
The abscissz are the pH of the gel at the end of the experiment, 
while the ordinates are the weight of the gelatin at the end of the 
experiment. All the values for the influence of the six monobasic 
acids, HCl, HBr, HI, HNOs, propionic, and lactic acid, on 
swelling lie on the same curve within the limits of the accuracy 
of the experiments, with a maximal weight of about 36 g., which 
is inside the variations for the controls with HCl referred to. 
Only acetic acid gives a slightly higher maximal value of about 






















50 

re 
E me on swelling 
£ - 
ee 
= 30 
= 25 ne : 
rae | 
7020 
2% ae 

10 

ne 

0 


6 


ow 
= 


td a0) 


pH 
Fie. 10.—Proof of valency rule for the influence of acids on the swelling of 
gels of gelatin. The influence of the seven monobasic acids is (aside from 
slight secondary effects of acids presumably on the cohesion of the gel) the same 
and considerably higher than that of the two dibasic acids. 


4 





42 g. at pH 3.2. The abnormal behavior of acetic acid does not 
occur in either membrane potentials or osmotic pressure, where 
the effects are due to isolated gelatin ions. The suspicion is, 
therefore, justified that the excessive effect of acetic acid on swell- 
ing is due to a diminution of the cohesion of the gel caused by the 
high concentration of acetic acid required to bring the pH to 
3.2 013.0. 

On the other hand, the strong dibasic acids, H,SO, and sulfo- 
salicylic acid, also act alike but cause a maximal weight of only 


53 Lors, J. and Kunitz, M.: Loc. cit. 


) eco heal 


f 
‘ 
. 





BEHAVIOR OF PROTEINS 61 


18 g., which is about one-half of the maximal weight of the gelatin 
under the influence of HCl. This ratio of 1:2 for dibasic and 
monobasic acids is about the same as that observed for the valency 
effect of anions in the case of osmotic pressure. The maximum 
lies at a pH of about 3.0 to 3.2 of the gel. 

Figure 11 shows the effect of weak dibasic and tribasic acids on 
swelling. From what has been said concerning the electrolytic 
dissociation of these acids it is obvious that their effect on swelling 
is also as clearly a confirmation of the valency rule as is their 
action on membrane potentials and on osmotic pressure. Such 


02 fom {Succinicacid| | | 
Da 








. [=75 
: bh i eda 
DD fx} ‘ 
: SONS 
c aS D 
ats 
fe) 
ms) 
kee 
Oo 
= 
nie 
oP) 
2 
= 


06 1B 0 22 24 26 28 50 32 34 36 38 40 4D Ah AG 48 
pH 


Fria. 11.— Influence of weak dibasic and tribasic acids on swelling. 


an influence of valency without influence on the chemical nature 
of the anion of an acid occurs only in properties which depend 
upon the Donnan equilibrium. The fact that the influence of 
acid on swelling is an osmotic effect explains why the curves 
representing this influence are similar to the curves representing 
the influence of acids on osmotic pressure. 

Viscosity.—It may seem strange that the influence of electro- 
lytes on the viscosity of certain protein solutions should be 
explained in the same way, but this seems to be the case. There 
are two types of viscosity, one type which holds for all kinds of 
solutions and one type which is specifically colloidal. We are 
concerned only with the latter type of viscosity, which is of a com- 
paratively high order of magnitude. According to Einstein’s 


62 COLLOIDAL BEHAVIOR 


formula, the viscosity of an aqueous protein solution is a linear 
function of the relative volume of the solute occupied in the solu- 
tion, as expressed in the equation 


= no(1 + 2.5¢) 


where 7 is the viscosity of the solution, 7. that of pure water, and 
y the ratio of the volume of the solute to that of the solution. 
If, therefore, the addition of little acid to a 1 per cent solution of 
isoelectric gelatin increases the viscosity of the solution until 
a& maximum is reached, and if the addition of more acid depresses 
the viscosity again, it follows that the addition of acid changes 
the relative volume occupied by the gelatin in water. This is 
only possible by water being absorbed by the protein and the 
question is how to account for this absorption of water by 
the protein under the influence of acid. Pauli assumed that 
the ionized protein surrounds itself with a jacket of water, which 
is lacking in the non-ionized protein. If this were true, all the 
proteins and amino acids should show a similar influence of acid 
on the viscosity of their solutions. The writer found that no 
such influence exists in the case of amino acids and at least one 
protein, namely, crystalline egg albumin. If Pauli’s assumption 
were correct, there is no reason why crystalline egg albumin 
should not show an influence of acid on viscosity of the same order 
as that which is found in the case of gelatin. The difference 
between gelatin and crystalline egg albumin is that the former 
sets to a solid gel if the temperature is not too high, while the 
latter does not. The formation of a continuous gel in the gelatin 
solution is preceded by the formation of submicroscopic aggre- 
gates which occlude water and which are capable of swelling, and 
these aggregates or precursors of the continuous gel increase in 
size and number on standing. To test this idea the writer 
made experiments with suspensions of powdered gelatin in water 
and found that such suspensions of powdered gelatin had a 
much higher viscosity than a freshly prepared solution of gelatin 
containing the same quantity of gelatin. Thiswastobe expected, 
if the influence of acid on the viscosity of proteins is due to the 
swelling of submicroscopic particles of gel. It harmonizes with 
this fact that the viscosity of solutions of crystalline egg albumin 
is of a low order of magnitude, which was to be expected if solu- 


BEHAVIOR OF PROTEINS 63 


tions of crystalline egg albumin contain few or no micelles. It 
was found, moreover, that the viscosity of suspensions of pow- 
dered gelatin increased under the influence of acid or alkali in the 
same way as did the swelling of jellies or the osmotic pressure of 
gelatin solutions. The viscosities were measured at 20°C. When 
the suspension of powdered gelatin was melted, it was found 
upon rapid cooling to 20°C. that the viscosity was considerably 
lower and that the influence of acid had almost disappeared. 
By these and a number of similar experiments it was possible to 
prove that the similarity between the influence of electrolytes 
on the viscosity of gelatin solution and the influence of electro- 
lytes on osmotic pressure is due to the fact that the influence on 
viscosity in such cases is in reality an influence on the swelling 
of submicroscopic protein particles, z.e., a function of osmotic 
pressure. This proof was made complete by showing that 
there exists a Donnan equilibrium between powdered particles 
of gelatin and a surrounding weak gelatin solution. The reader 
is referred to the writer’s book and papers for further details.*4 

The Action of Salts.—A few words must suffice to explain the 
action of salts on the four colloidal properties of proteins. Salts 
do not raise the value of z but only of y in equations (1) and (2), 
and, as a consequence, salts can only diminish the excess in the 
concentration of the diffusible ions inside the protein solution 
or protein gel over that outside. This explains the~ purely 
depressing effect of salts on the values of the four colloidal prop- 
erties. It follows, furthermore, that only the anion of a salt 
should be able to influence the four colloidal properties of protein 
chloride; namely, membrane potentials, osmotic pressure, swel- 
ling, and that type of viscosity which depends on the swelling of 
submicroscopic particles of proteins. Furthermore, it follows 
that the valency only but not the chemical nature of the ions of a 
salt should have such a depressing effect. The limited space 
permits us only to show that this is true for the action of salts 
on osmotic pressure and swelling. 

In such experiments it is necessary to guard against the 
possibility of any change in the pH by the addition of the salt 
to the protein solution or the protein gel. The methods of avoid- 
ing this error are given in the writer’s book and more explicitly 

54 Lozs, J.: “Proteins,” p. 150. 


64 COLLOIDAL BEHAVIOR 


in a recent paper.®® Earlier workers had failed to measure the 
hydrogen ion concentration of their protein solutions and gels 
and to compare the effects of salts at the same pH of the protein 
solution or gel, and, hence, mistook the effect of variations 
of the pH (which they overlooked) for effects of the anion of the 
acid added. This gave rise to the myth of the so-called Hofmeis- 
ter ion series in colloidal behavior, according to which not only 
the valency but also the chemical nature of the ions of a salt 
are said to have an effect on the colloidal behavior of proteins. 
This statement is altogether incorrect insofar as it applies to 
those properties of proteins which depend on the Donnan 
equilibrium, such as membrane potentials, osmotic pressure, 
swelling, and that type of viscosity which is due to the 
swelling of submicroscopic particles, since these properties 
are affected only by the valency and not by the chemical 
nature of the ion. Crystalloidal properties of proteins, however, 
such as solubility, cohesion, diffusion potentials, etc., depend, 
of course, not only on the valency but also on the chemical 
nature of the ions of a salt. As long as the colloid chemists 
will continue in their failure to measure and to consider the pH 
of their solutions, and as long as they will continue to struggle 
against the acknowledgment of the fact that the Donnan equilib- 
rium is the basis of the strictly colloidal behavior (at least as far 
as the proteins are concerned), so long will they fail to understand 
that the Hofmeister ion series (for the four colloidal properties 
of proteins) are not only experimental errors but theoretically 
impossible. They will also fail to understand that the chemical 
nature of the ions of a salt must play a réle in crystalloidal beha- 
vior while it cannot play a role in the colloidal properties which 
depend on the Donnan equilibrium. 

Figure 12 gives the influence of seven salts, NaCl, NaBr, 
NaNO;, Nal, NaCNS, Na acetate, and NaeSOuz, on the osmotic 
pressure of a solution of gelatin chloride of pH 3.8, special care 
being taken that the pH was not altered by the addition of salt, 
through measurements of the pH with the hydrogen electrode.°*® 
The depressing effects of the six salts with monovalent anion 

% Lorn, J.: “Proteins,” p. 99; Lorn, J. and Kunitz, M.: J. Gen. Physiol., 


5 (1922-23), 693. 
56 Lons, J. and Kunitz, M.: J. Gen. Physiol., 5 (1922-23), 701. 


BEHAVIOR OF PROTEINS 65 


(NaCl, NaBr, NaI, NaNO;, NaCNS, Na acetate) on the osmotic 
pressure of the gelatin chloride solutions of pH 3.8 (containing 1 
g. dry weight of originally isoelectric gelatin in 100 cc.) lie, within 
the limits of experimental accuracy, on one curve, which is entirely 





0 Bi a ibe sts Wz 2 25 28 ot 2 i : i 


Concentration 


Fie. 12.—All salts with monovalent anions depress the osmotic pressure of 
gelatin chloride solutions of pH 3.8 to the same extent (within the limits of 
experimental accuracy). Na2SO,4 depresses considerably more. 


different from the curve for the effect of Na2SO.. The osmotic 
pressures are a little over twice as high when the anion of the salt 
is monovalent than when it is divalent. The variations in the 
effects of the six salts with monovalent anion are chance varia- 


66 COLLOIDAL BEHAVIOR 


tions, since they are also found when no salt is added, 7.e., at 
concentration 0 in Fig. 12. 

This shows that all the salts with monovalent anions have the 
same effect on the osmotic pressure when the pH is kept constant 
and that the so-called Hofmeister anion series is based on error. 
The anion series must be replaced by the valency rule. This 
statement is supported by experiments on the influence of Naz 
oxalate, Nae tartrate, and Nae succinate upon the omostic pressure. 
The effect of these salts lies between that of Na2SO, and NaCl 
as the valency rule demands. 


es 


on swellin 
gelatin chloride 


pH 3.8 | 


eee 
Influence oe MDa 


Weight of gelatin i 





Y 8192 4096 2048 1024 5SI2 256 128 64 32 16 8 4 2 
Concentration of Cl ions 


Fic. 13.—All chlorides depress the swelling of a gelatin chloride gel of pH 3.8 
to the same extent at the same concentration of Cl ions. 


Figure 13 gives the influence of five chlorides, KCl, NaCl, 
LiCl, CaClz, LaCls, on the swelling (measured by weight of the 
gel) of a gel of gelatin chloride containing 1 g., dry weight, of 
originally isoelectric gelatin. The pH of the gel at equilibrium 
was 3.8. The abscisse are the concentrations of the Cl ions of 
the salts and the ordinates the weight of the gel. It is obvious 
that all five salts depress the swelling equally at the same con- 
centration of Cl and that, hence, the cation of the salt has no effect. 

The next fact to be ascertained was whether or not only the 
valency of the anion of the salt is of influence or whether the 
anion series generally quoted in colloidal literature is valid, 
according to which the swelling is a maximum in NaCNS§, and a 


BEHAVIOR OF PROTEINS 67 


minimum in Na acetate (leaving the divalent anions out of 
consideration for the present). 

Seven salts with monovalent anions were tried, namely, NaCl, 
NaBr, NaI, NaNO;, NaCNS, Na acetate, and Na lactate. The 


fs on swell in 

= ees enon p38 
if Y © §—* — Ten SG 
ce 

ae ne lactate 2 aaiee 

43 Nal 

mop) 

Vv 

= 





> MMMMMMMMM MM M 


8192 4096 2048 1024 512 256 128 64 32 16 8 4 
Concentration 
Fig. 14.—All salts with monovalent anions depress the swelling of a gelatin 
chloride gel to the same extent (within the limits of experimental accuracy) at 
pH 3.8 


Weight of gelatin in gm. 





Concentration 


Fie. 15.—Na2SO. depresses the swelling of a gelatin chloride gel considerably 
more than NaCl. 


results are given in Fig. 14. It is obvious that the effects of all 
of these seven salts lie on one curve, and that the variations are 
essentially the chance variations due to the limits of experimental 
accuracy. This is proved by the fact that the same variations 


68 COLLOIDAL BEHAVIOR 


are observed when the concentration of salt is zero, 7.e., when no 
salt is added. ‘There is not the slightest indication of the Hof- 
meister anion series. Slight influences of the salts on the cohe- 
sion of the gel of gelatin may exist, but they are too small to 
play a réle. 

While salts with monovalent anions have the same depressing 
effect for the same concentration of anions, salts with bivalent 
anions have a much greater depressing effect on swelling than 
salts with monovalent anions. This is illustrated in Fig. 15, 
showing the difference in the effect of equal molar concentrations 
of NaCl and Na2SO, on swelling. NaCl does not depress swelling 


in concentrations of rie or below, and the depressing effect 
? 


of NaCl on the swelling of gelatin chloride of pH 3.8 commences 
to be noticeable at a concentration of ri This is true for 


all salts with monovalent anions, as Fig. 14 shows. NasSO, 


begins, however, to depress at a concentration between Ta 


and Tai and the curve for the SO, effect drops much more 


rapidly to the minimum than in the case of NaCl. Since, how- 
ever, the degree of swelling of a gel does not only depend on the 
osmotic pressure of the solution inside the particle but also upon 
the force of cohesion, and since the cohesion may also be influenced 
by the electrolytes added, it is necessary to guard against a 
confusion of the two possible effects of an electrolyte. These 
effects on cohesion are especially noticeable in high concentrations 
of electrolytes. The cohesion effect of electrolytes has nothing 
to do with the Donnan equilibrium and, hence, in such cases an 
influence of the chemical nature of the efficient ion may be 
observed. Such cohesion effects may appear especially in higher 
concentrations of electrolytes and this explains some of the 
statements in colloid literature. 

Summarizing all the results, we can say that the membrane 
potentials, osmotic pressure of gelatin chloride solutions, or the 
swelling of gelatin chloride gels, and that type of viscosity of 
gelatin chloride solutions which depends on the swelling of 
submicroscopic solid particles in the solutions are affected only 


BEHAVIOR OF PROTEINS 69 


by the anion but not by the cation of a salt; that all anions of 
the same valency have the same depressing effect on these four 
properties of gelatin chloride, and that the depressing effect is 
greater for the divalent than for monovalent anions. Such a 
result is possible only for properties depending on the Donnan 
equilibrium. 

The total result of these investigations is that it is incorrect to 
distinguish between colloids and crystalloids—at least as far as 
the proteins are concerned—but that we must distinguish 
instead between colloidal and crystalloidal properties. Proteins 
are crystalloids, both in regard to their chemical reactions and 
their solubility, but on account of the large size of their ions they 
easily fulfill the condition for the establishment of a Donnan 
equilibrium, namely, that the protein ion is prevented from dif- 
fusing through membranes or gels which are easily permeable to 
the smaller crystalloidal ions. Those properties of the proteins 
which depend on the Donnan equilibrium constitute their 
colloidal behavior. For the sake of convenience, we may con- 
tinue to distinguish between colloids and crystalloids, but in 
that sense only that colloids possess large ions, the diffusion 
of which is blocked by dialyzing membranes permeable to the 
smaller ions of typical crystalloids. There is, however, no 
further justification for any distinction between the chemistry 
or solubility of colloids and of crystalloids, as far as the proteins 
are concerned. That the rdle of the Donnan equilibrium for the 
colloidal behavior of proteins had been overlooked was again due 
to the fact that the majority of the workers in this field never 
measured the hydrogen ion concentration of their protein 
solutions or gels. Without such measurements it was impossible 
to notice the réle which the Donnan equilibrium plays in these 
phenomena. 


CHAPTER III 


THE FLOCCULATION AND STABILITY OF COLLOIDAL 
SUSPENSIONS 


By 


JoHN H. NorTHrRop 


One of the most striking characteristics of suspensions of finely 
divided matter is the fact that under certain conditions the 
individual particles remain discrete, whereas under other condi- 
tions they collect into larger aggregates. Under the former con- 
ditions the rate of settling is very slow and the suspension may 
be returned to its original condition by mechanical disturbances, 
whereas under the latter the particles settle rapidly and, in 
general, cannot be made to separate and return to their original 
state. This peculiarity of such systems is of great theoretical 
and practical interest and has been the subject of a very large 
number of papers. It is the fundamental phenomenon concerned 
in the formation of deltas by the sedimentation of river silt in 
the ocean, of the production and use of colloidal fuels and emul- 
sions, the agglutination of bacteria, and innumerable other com- 
mon processes. It is impossible in so short a space as can be 
devoted to this chapter more than to outline the clearest cut 
results. General reviews covering the entire field may be found 
in the books of Freundlich,! Burton,? Bancroft,? and Taylor. 

A suspension undergoing the process of flocculation presents a 
definite series of changes. At first the individual particles cannot 
be seen except with the microscope. Their presence is shown, 
however, by a Tyndall cone when light is passed through the 
suspension. Larger particles then make their appearance and 

* FreunpbuIcu, H.: “Kapillarchemie,” 2nd ed., Leipzig, 1922. 

* Burton, E. F.: “The Physical Properties of Colloidal Solutions,” 2nd 
ed., London, New York, Bombay, Calcutta, and Madras, 1921. 

*Bancrort, W. D.: “Applied Colloid Chemistry,’ New York and 
London, 1921. 

*'Taytor, W. W.: “Chemistry of Colloids,” 3rd ed., New York, 1915. 

70 


COLLOIDAL SUSPENSIONS ia 


may usually be seen as discrete clumps. These clumps consist, 
in general, of a number of small particles adhering firmly together 
but still retaining their individual form. More or less rapid 
settling of these larger particles now occurs and in the course of 
time the solid matter forms a precipitate on the bottom of the 
vessel, leaving a clear liquid above. The suspension is now 
flocculated or ‘‘sedimented.”’ The appearance under the micro- 
scope is similar, except that, in addition, it can be seen that the 
small particles are in rapid irregular movement—the Brownian 
movement—whereas the large clumps are stationary. It can 
also be usually seen that the particles do not actually coalesce 
but merely approach one another closely. (In the case of the 
‘‘breaking”’ of an emulsion there is actual coalescence; this is a 
distinct phenomenon and will not be considered here.) 

It may be seen from the above brief description that the 
phenomenon can be divided into two distinct steps: first, the 
collection of the small particles into larger aggregates, and, 
second, the settling of these aggregates to the bottom of the vessel. 
In regard to the latter effect, the small and large particles differ 
from each other both in the rate of settling and in the final condi- 
tion of equilibrium, although under ordinary conditions the 
difference in the rate is of the greater significance. 


EFFECT OF THE SIZE OF PARTICLES ON THE RATE OF SETTLING 


The formula for the steady rate of fall of a small body in a 
viscous medium was given by Stokes as 


“a? (D —d)g 
2 eae (1) 


Z 

where a is the radius, D the density of the particle, d the density 
of the solution, z the viscosity of the solution, and g the accelera- 
tion due to gravity. This formula was tested by Perrin® for 
small particles by comparing the radius calculated from the rate 
of fall with that determined by direct measurement or calculated 
from the weight and size. 


RapDivus IN » DETERMINED BY 


DIRECT FRoM 
MEASUREMENT W EIGHING StToxEs’ Law 
0.371 0.3667 Rei es 


5 PERRIN, J.: ‘‘Die Atome,”’ Dresden, 1914, p. 90. 


72 COLLOIDAL BEHAVIOR 


The experiment shows that the particles obey Stokes’ law with 
the greatest exactness. This result is of special importance, 
since the validity of Stokes’ law is assumed in all calculations 
concerning the Brownian movement. It follows, therefore, that 
the speed of settling of different size particles, other conditions 
being the same, will increase with the square of the radius and 
the difference in rate between visible and microscopic particles 
will be enormous. In Perrin’s experiments the rate was a few 
millimeters a day. 


EFFECT OF THE SIZE OF PARTICLES ON THE FINAL EQUILIBRIUM 


The English botanist, Brown, noted that pollen grains as seen 
under the microscope possessed rapid irregular movements. 
This peculiar constant motion has become known as the Brownian 
movement. It was soon found that the motion was independent 
of the nature of the particles and could not be ascribed to any 
outside influence. It is less in viscous liquids and very rapid 
in gases. The motion is less in large particles. It follows from 
the doctrine of equipartition of energy that the mean kinetic 
energy (14 mv.”) of the particles must remain constant. The 
velocity decreases rapidly, therefore, as the size increases.® 
Svedberg has shown that it is not affected by the potential of the 
particle nor by the addition of electrolytes.’ It was suggested 
by Wiener that this motion was due to the bombardment of the 
particles by the molecules of the solvent. The motion, therefore, 
becomes strictly analogous to the kinetic motion of the molecules 
themselves. A quantitative theory for this motion was worked 
out independently by Einstein and by v. Smoluchowski and verified 
experimentally by Perrin. The part of the theory which is of 
interest in this connection is the prediction regarding the final 
distribution of the particles at equilibrium. If the Brownian 
movement is really analogous to the kinetic motion of gases, 
then the distribution of the particles at equilibrium should be 
determined by the same law that regulates the density of a gas at 


§Lewis, W. C. McC.: “A System of Physical Chemistry,” London, 
New York, Bombay, Calcutta, and Madras, 1920, vol. 1, chap. I. 

7For a thorough discussion of the Brownian movement, see BuRTON, 
K. F.: “The Physical Properties of Colloidal Solutions,” p. 50; Pmrrin, J.: 
“Die Atome,”’ Dresden, 1914, p. 83; Freunp.ticu, H.: “ Kapillarchemie,”’ 
2nd ed., Leipzig, 1922, p. 469. 


COLLOIDAL SUSPENSIONS 73 


different levels. Equilibrium will be established when the effect 
of gravity exactly equals the osmotic pressure (in this case the 
Brownian movement) of the particles or molecules. In the 
case of gases this formula is 


gM p 
where h is the height, p, the pressure at the bottom of the column, 
p the pressure at height h, g the acceleration due to gravity, and M 
the molecular weight.? Since the osmotic pressure is proportional 


to the number of particles per unit of volume, the formula, as 
applied by Perrin to suspensions, becomes 


etal No 


h=- 4 In 
gNarr*(D — d) ‘ 





(2) 


in which N is Avogadro’s number, D is the density of the particle, 
and d the density of the liquid. 

The formula shows that the height necessary to give any relative 
pressure compared to the pressure at the bottom varies inversely 
as the weight of the particles. If the weights of the particles 
are as 100,000,000 to 1, then the height at which the pressure will 
be half that of the bottom pressure will be in the same ratio, 7.e., 
if a particle 100,000,000 times as heavy as an oxygen molecule is 
compared to oxygen, the height at which the pressures (or con- 
centration of particles) is halved will be as 50yu to 5,000 meters.? 
The formula was tested by Perrin by measuring the concentration 
of particles at various heights and determining the diameter and 
density of the particles as well as the density of the liquid. 

He found?® that the concentration of particles decreased in 
geometric proportion as the height increased in arithmetic pro- 
portion, as the following figures show. 


HIGHT RELATIVE NUMBER 
IN pu OF PARTICLES 
5 100.0 
35 47.0 
65 22.6 
95 12.0 


8 FREUNDLICH, H.: ‘‘Kapillarchemie,”’ 2nd ed., Leipzig, 1922, p. 469. 
® PorRRIN, J.: “Die Atome,”’ Dresden, 1914, p. 94. 
10 Tbid., p, 9b. 


74 COLLOIDAL BEHAVIOR 


This is the relation predicted. It follows that at equilibrium 
the number of particles of microscopic size at any appreciable 
height from the bottom would be very small. 

The critical test of the theory, however, was the calculation of 
Avogadro’s number N. The accepted value for this constant 
from measurements with gases is 61 X 102, whereas Perrin found 
68 < 1022. The agreement is astounding when it is remembered 
that the measurements were made on particles approximately 
100,000,000 times the mass of gas molecules. 

Perrin’s experiments leave little doubt that the relation between 
the size of the particles, the rate of settling and the final 
distribution is accurately expressed by formulas (1) and (2). 
If the necessary data regarding the size of the particles, the vis- 
cosity of the solution, etc., are known, it is, therefore, possible to 
calculate both the rate of fall of the particles and the final state of 
equilibrium. Briefly, it may be said that if the size alone is 
varied, the rate at which the particles fall will increase as the 
square of the radius and that at equilibrium the distance from the 
bottom, at which the concentration of particles will be halved, 
will be inversely proportional to the mass. The difference in 
the behavior of suspensions before and after the formation of 
larger aggregates is, therefore, clearly accounted for, and it only 
remains to determine what forces or conditions regulate the for- 
mation of agglomerations of particles. 

If a stable suspension is observed under the microscope, it may 
be seen that, although the particles approach each other, they 
do not actually collide. If some substance is now added which 
precipitates the suspension, the particles then collide and stick 
together. Since they sometimes adhere to each other and some- 
times remain separate, there must evidently be a force which 
tends to keep them apart and another force which holds them 
together. Ifthe repulsive force is greater than the cohesive force, 
or greater than the momentum of the particles due to their move- 
ment, the particles will remain separate, whereas if it is less they 
will adhere into larger aggregates. 

It has long been known that particles or surfaces in contact with 
liquids are electrically charged, since under the influence of an 
external e.m:f. the particles move. If the liquid moves while the 
solid is kept stationary, the phenomenon is known as electro- 


COLLOIDAL SUSPENSIONS 15 


endosmosis, whereas if the particles move through the liquid, it is 
known as cataphoresis.!!_ It was early suggested that it was this 
repulsion, due to the charge carried by the particles, which pre- 
vented their touching each other, and practically all theories of 
the stability of suspension depend in some way on this potential 
difference between the particles and the surrounding liquid. 
In order to trace the connection between this property and the 
behavior of the particles, it is necessary to touch somewhat on the 
nature and origin of this potential difference. 


METHOD OF MEASUREMENT AND PROBABLE NATURE AND ORIGIN 
OF THE CHARGES OF COLLOIDAL SUSPENSIONS 


All methods of measurement of the charges carried by colloidal 
suspensions depend on determining the motion of the particles 
in an external field. This may be done either by noting the 
movement of the boundary of the suspension as a whole in a 
U-tube,!’ or by following the motion of a single particle under the 
microscope or ultramicroscope. (A convenient type of apparatus 
for these measurements has been described by the writer.)* 

The potential between the surface of the particle and the sur- 
rounding film of liquid may then be calculated by the Lamb- 


Helmholtz formula. 
id KX 

in which 7 is the viscosity, K the dielectric constant of the 
surface layer, v the velocity in centimeters per second, and X the 
potential gradient. All electrical units are electrostatic. Sub- 
stituting the viscosity of water and the dielectric constant at 20°C. 
and changing to millivolts, the formula becomes 

mw per second 
Volts per centimeter 
This formula was derived on the assumption that each particle 





P.D. in millivolts = 13 


11 Cf, FREuNDLIcH, H.: “‘Kapillarchemie,” 2nd ed., Leipzig, 1922, p. 326; 
Burton, E. F.: ‘The Physical Properties of Colloidal Solutions,” 2nd ed., 
1921, p. 125. 

12 Burton, E. F.: “The Physical Properties of Colloidal Solutions,”’ 
2nd ed., p. 131. 

13 Norturop, J. H.: J. Gen. Physiol., 4 (1921-22), 629; Norrurop, J. H. 
and CuLuEn, G. E.: J. Gen. Physiol., 4 (1921-22), 635. 


76 COLLOIDAL BEHAVIOR 


acts like a small condenser and is surrounded by a Helmholtz 
double layer. The charge on the particle as a whole, including 
the film of liquid, is, therefore, 0. A potential difference exists, 
however, between the two oppositely charged layers and it is 
between these layers that the motion occurs. It follows that 
the value for the dielectric constant of the liquid between the two 
layers should be inserted in the formula. There is no way of 
determining this value, however, so that the dielectric constant of 
the pure liquid is usually used. It has been found by numerous 
investigators that this formula is experimentally correct so farasit 
concerns the relation between the rate of migration and the 
impressed e.m.f., or the viscosity. There is, however, no 
evidence concerning the correctness of the actual value of the 
potential calculated from the velocity. It was shown by Hardy 
that the size of the particles was without effect on the observed 
motion, which also agrees with the theory. 

The theory of Helmholtz and Lamb gives us no information as 
regards the origin of this potential difference and no satistactory 
theory has been suggested up to the present. It has been shown 
by Haber and Klemensiewicz, that the cataphoretic potential 
is not the same as the Nernst electrode potential. This has been 
ascribed by v. Smoluchowski to the fact that the p.p. between the 
interior of the particle and the liquid is the Nernst potential, 
whereas the cataphoretic p.D. is between the movable and fixed 
filras. 

McTaggart’s experiments with air bubbles and Lenard’s 
measurements of waterfall electricity indicate that the source of 
the potential may be entirely in the film of liquid surrounding the 
particle.1® 7 

Wilson,!” on the other hand, suggested that the p.p. was due to 
a Donnan equilibrium, and Loeb!’ has found that there is some 
analogy between the two but that quantitatively they are differ- 
ent. This theory accounts satisfactorily for the observation 


14 Burton, E. F.: ‘The Physical Properties of Colloidal Solutions,” 2nd 
ed., p. 137. 

16 FRpuNDLICH, H.: “ Kapillarchemie,” 2nd ed., Leipzig, 1922, p. 341. 

16 Cf, Lorn, J.: J. Gen. Physiol., 5 (1922-23), 515. 

17 Wiison, J. A.: J. Am. Chem. Soc., 38 (1916), 1982. 

18 Lorn, J.: J. Gen. Physiol., 5 (1922-23), 515. 


COLLOIDAL SUSPENSIONS id 


that the sign of charge changes at the isoelectric point of the 
particle when the latter is amphoteric and accounts also for the 
relation between the ionization and the sign of charge. It is 
known, for instance, that substances which tend to dissociate 
as acids are usually negative and basic substances positive. 

v. Hevesy!® considers the particles as analogous to large ions. 
It is true that the rate of migration is about the same. There 
would seem to be a definite difference, however, between the 
mechanism by which the solution as a whole is kept electrically 
neutral. In the case of an ion there is always an equal number of - 
ions of the opposite charge in the solution, whereas, according to 
the Lamb-Helmholtz theory, each particle as a whole is electri- 
cally neutral. In any case, it is certain that the p.p. of the parti- 
cles is closely connected with the presence of electrolytes in the 
solution. It has been found by numerous workers that the more 
carefully the solution was freed from electrolytes the lower the 
potential and the more unstable the suspension.”° 

The general opinion at present appears to be that the charge is 
conferred by the combination of the particle with an ion, although 
the nature of this combination is uncertain. 

Whatever the source of the potential on the particles, it 
follows that similarly charged particles would tend to repel each 
other and thereby: render the suspension stable. If, however, 
the particle as a whole is electrically neutral, this repulsion would 
not obey Coulomb’s inverse square law, but would only become 
effective when the particles approached each other so closely 
that the outside of the double layers overlapped. This conclu- 
sion is borne out by Perrin’s observation?! that the distribution 
of charged particles is abnormal when the distance between them 
is less than about 1.7 times the radius. When the charge is 
removed, this anomalous distribution disappears. This experi- 
ment furnishes strong evidence that the particles are held apart 
by their electric charge. 

It was first noted by Hardy”? that suspensions of denatured 
proteins were most unstable at the isoelectric point, and he sug- 


19 vy, Hevesy, G: Kolloid-Z., 21 (1917), 129. 

20 Beans, H. T. and Eastuack, H. E: J. Am. Chem Soc., 37 (1915), 2667. 
21 PHRRIN, J.: Compt. rend., 158 (1914), 1168. 

22 Harpy, W. B.: Proc. Roy. Soc., 66 (1900), 110. 


78 COLLOIDAL BEHAVIOR 


gested that this was due to the fact that they were electrically 
neutral at this point. This conclusion was verified qualita- 
tively by a large number of workers.?? A number of cases were 
found, however, in which there seemed to be no direct connection 
between the stability and the potential. 

Ellis? made a number of measurements on oil emulsions and 
found that the stability was closely connected with the potential. 
The experiments were then carefully carried out by Powis,”® 
who made accurate measurements of the p.p. between the oil 
drops and surrounding liquid in a series of electrolyte solutions 
and found that whenever the potential between the drops and the 
surrounding liquid was reduced below about 30 millivolts, the 
particles collected into larger aggregates. A summary of Powis’ 
results is given in Table I. They leave little doubt that in this 
case the potential is the decisive factor. It will be noted, how- 
ever, that it becomes necessary to assume a critical p.p. instead 
of Hardy’s zero potential. Powis found later that in the case of 
arsenic sulfide suspensions this critical p.p. was different for 
different salts. (This effect will be discussed below in connection 
with work on bacterial agglutinations.) A number of experi- 
ments were carried out by the writer on suspensions of typhoid 
bacteria. It was found that when these had been treated with 
an excess of antiserum they behaved in the same way as Powis’ 
oil drops, 7.e., agglutination always occurred whenever the poten- 
tial was reduced below a critical value of about 15 millivolts. A 
summary of these experiments is shown in Fig. 1. 

A similar series of experiments was carried out by Loeb?® on 
suspensions of collodion particles. The results aresummarized in 
Table II. Here again there is no doubt that agglutination occurs 
whenever the p.p. is reduced below about 15 millivolts. The 
same result was obtained when the particles were previously 
treated with egg albumin, and with particles of denatured egg 
albumin.2”7 These experiments leave little doubt that in these 
cases the potential is the determining factor for the stability of 

23 Burton, E. F.: ‘The Physical Properties of Colloidal Solutions’’ 
2nd ed., 1921, p. 149. 

24 Huis, R.: Z. physik. Chem., 78 (1911-12), 321; 80 (1912), 597. 

% Powis, F.: Z. physik. Chem., 89 (1914-15), 91, 179, 186. 


2 Lorn, J.: J. Gen. Physiol., 5 (1922-23), 123. 
27 Tbid.: 485. 


a 





79 


COLLOIDAL SUSPENSIONS 


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80 COLLOIDAL BEHAVIOR 


























TABLE [28 
Salt 
Concen- 
tration, KCl BaCle AlCls ThCla4 
millimol/ 
liter $ 
ia be Relative | P.p., Sta- P.D., Sta- P.D., Sta- 
potential, | stability | mv. | bility | mv. | bility | mv. | bility 
— 46 1.00 — 46 1.00 —46.0) 1.00 — 46.0 130 
OOO5 No oe SB Satta) TS es Sorted ee ok |e 1.00 — 39.0 1.0 
0.010 — 38.0] 0.80 — 6.5 0.3 
020207 | few kice OU! A BANS eG ei aree oltmese et an rma 0.35 — 8.5 0.2 
0.050 —17.0| 0.35 +29.0 1.0 
0.100 an ee ed ee ee A hs oe A 0 
0.200 — 50 1.00 — 413 1.00 = 8.0) SO%SC Re 2 120 
0.500 Ra i Bae Bice akEoce Galleries Stee +52.0 {0 
1.000 — 59 1.00 — 30 0.75 + 2.5} 0.30 
2.500 —61 1.00 — 25 0.45 
5.000 — 51 00 it eo oR RES)! Cae See +23.0 0.4 
10.000 Lae 3.5, 0.40 
20.000 — 37 Ox GO) A Pl ) ee eee +17.0| 0.4 
25 .000 see — 8 0.40 
100 .000 — 22 0.60 ahs 0.35 5-OF 20°60 
200 .000 —12 0.50 + 1 0. 4024 ate SESE + 7.0) 0.6 
500 .000 — 8 0.25 es poe 5.0} 0.60 
700 .000 Saas Ye 5 oe + 4 0.10 
1,000 .000 0.30 
1,500 .000 0.15 























the suspension. It is possible to predict from a measurement of 
the cataphoretic p.p. alone whether or not the suspension will 
remain stable. It also follows that the cataphoretic potential 
is the decisive one for the prevention of agglutination and, 
further, that this potential must be directly proportional to the 
rate of migration in the electric field as predicted by the Lamb- 
Helmholtz formula.”® 


28 Powis, F.: Z. physik. Chem., 89 (1914-15), 191. 
29 Cf. Lons: Loc. cit. ; 


COLLOIDAL SUSPENSIONS 


81 


TABLE II .2°—CaTapHoReTIC CHARGE AND STABILITY OF SUSPENSIONS 
OF PARTICLES OF COLLODION 





oa Cle. © @ 6.6 8 


Spee vet SS .8) lay 6 


Swine (8) a8 v8: e 


vite © 86s & 6 « 


i Ta are, 6) 6) |e Ye se 


9 Oe 8 Se a « % 


fey B) Ceneiniork iy. 


ee i®, Oem ws 8, es 


NasFe(CN).. SARS 


30Lors, J.: J. Gen. Physiol., 5 (1922-23), 123. 


— RM) goa a = eed 


ner ey ese dee he 




















2 3 4 5 
Minimum P.D. Maximal P.D. 
concentration in concentration in 
required for milli- at which milli- 
precipitation volts suspension volts 
| remains stable 
pH 5.8 
; | 
u/2 (10) m/4 17 
M/2 10 M/4 14 
M/4 14 M/8 21 
mM/4 13 M/8 19 
M/16 iby M/32 21 
M/16 a M/32 15 
M/16 15 M/32 19 
M/32 14 m/64 17 
M/2,048 14 m/4,096 21 
PULL 
M/2 M/4 18 
M/4 Ly M/8 20 
M/16 16 M/32 24 
M/32 15 m/64 19 
pH 3.0 
M/2 7 M/4 | 14 
M/4 12 M/8 (Lost) 
M/32 16 m/64 19 
M/2,048 14 m/4,096 18 
| mM/4 mM/8 14 





























82 COLLOIDAL BEHAVIOR 


PRECIPITATION BY NON-ELECTROLYTES AND THE MuTuau PRE- 
CIPITATION OF OPPOSITELY CHARGED COLLOIDS 


It was noted by Linder and Picton*! that two oppositely charged 
suspensions would precipitate each other if they were mixed in 
proper proportion. If either component were present in excess, 
the suspension again became stable. These observations have 
since been extended and confirmed for a large number of sub- 
stances. In general, suspensions of the same charges do not 
precipitate each other. It has usually been assumed that this is 
also an electrical phenomenon and that agglutination occurs 
owing to the neutralization of the charges. As Bancroft*? 


——Agglutination +t to C 


— 
—=—_ «== 


Potential in millivolts 





Fig. 2.—Agglutination of bacillus of rabbit septicemia by egg albumin at differ- 
ent pH. 


has pointed out, however, the effect is not purely a neutralization 
one, since the relative order of flocculation of a series of positive 
suspensions by a series of negative suspensions is not always the 
same. It is evidently necessary to consider the combination as 
separate from the neutralization. It is possible also that the 
difference may be partially due to a difference in the critical 
potentials. Figure 2 represents the results of a series of experi- 
ments in which a suspension of bacteria was agglutinated by 
the addition of egg albumin. The figure shows that the agglutina- 
31 LInDER, S. E. and Picton, H.: J. Chem. Soc., 61, 67, 71, 87. 


32 BancrorT, W. D.: ‘Applied Colloid Chemistry,” New York and 
London, 1921, p. 226. 


83 


COLLOIDAL SUSPENSIONS 


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84 COLLOIDAL BEHAVIOR 


tion is again determined solely by the potential. It also shows 
that the potential of the particles is affected even where the 
protein has the same sign of charge as the particles, since at pH 3 
the addition of the (positive) egg albumin renders the organisms 
still more positive. The fact that the effect of the potential 
is the result of the combination rather than the cause is clearly 
shown in Fig. 3, in which the amount of “agglutinin”? combined 
with the bacteria at various pH is compared with the amount 
required to agglutinate. According to Michaelis, the antibody is 
positive on the acid side of pH 5.0, so that a maximum effect 
would be expected between this point and pH 3. This is the case 
with the agglutination, but no difference is noticeable in the 
amount of antibody combined. It will be noted that the albumin 
stabilizes the suspension in some pH ranges instead of agglutinat- 
ing and that this effect is also due to the potential. A protective 
colloid and precipitating colloid can, therefore, not be separated.** 
The addition of proteins, etc., sometimes has a marked effect on 
the properties of a suspension by affecting the critical potential. 
This effect will be discussed more fully below. 

As far as the writer is aware, there are no complete potential 
measurements in the case of the mutual precipitation of two 
oppositely charged suspensions, so that it cannot be determined 
whether or not this is also purely a question of lowering the 
potential below the critical value. Linder and Picton, however, 
showed that in the presence of an excess of one of the components 
the entire suspension was stable and had the charge (qualitatively) 
of the excess suspension, and that the flocculated suspension had a 
lower p.p. than either of the suspensions alone. 

In the experiments discussed so far it has been possible to 
predict whether or not the suspension would precipitate simply 
by measuring the potential after the substance under investiga- 
tion had been added. There are a number of cases, however, 
where this is not the case, and it is necessary to measure another 
variable. This has always been a marked property of bacterial 
suspensions. It was noted by Neisser and Friedemann* that 


°° FREUNDLICH, H. and LoEnina, E.: Fest. Kaiser Wilhelm Ges. Férderung 
Wiss., 10 jahr. Jub., 1921, 82. 

** Neisser, M. and FrimpemMann, U.: Muinch. med. Wochschr., 1904, 
No. 19. 


COLLOIDAL SUSPENSIONS 85 


suspensions of typhoid bacilli were not agglutinated by mono 
and divalent salts as were suspensions of the organism treated by 
immune serum. This observation was confirmed by Buxton.*5 
The experiments were repeated by the writer*® and the potential 
measurements were made. ‘The results were the same as those 
found by Neisser and Friedemann. The experiments are 
shown graphically in Figs. 4 and 5. Figure 5 shows that in 
all cases where the potential was lowered to less than about 13 
millivolts by a concentration of salt of less than 0.01 N, aggluti- 
nation occurred, as would be the case with the suspensions already 


ae See 
Le 
fee: 5: 


+13 






2 0 0 
2 | 
E-g 108 
fe, 
5 
6 20 -2.0 
-39 lex ois -3, 
CeCe i 1082 10 10°! 1.0 10.0 : 


Acid concentration, equivalentsper liter 


Fia. 4.—Effect of acid concentration on the potential and agglutination of 
suspension of B. typhosus. 


discussed. If, however, the concentration of salt required to 
lower the poteatial to this value was 0.01 m or more, this was no 
longer true and no complete agglutination occurred, even though 
the potential was reduced to zero. It is evident that either (1) 
the potential measurements are wrong, or (2) some other factor 
has to be considered beside the potential, since with this suspen- 
sion it is no longer possible to predict the flocculation from the 
potential alone. The fact that in the earlier experiments the 
potential measurements agreed so well with the flocculation 
results, even in high salt concentrations, rendersit very improbable 

35 Buxton, B. H.: Z. physik. Chem., 57 (1907), 47. 

36 Norturop, J. H. and Dr air P. H.: J. Gen. Physiol., 4 (1921-22), 
639. 


86 COLLOIDAL BEHAVIOR 


that these measurements are at fault. It may be shown directly 
that this is not the case by suspending the organisms in various 
concentrations of acid with various total salt concentrations. 
It is found in this experiment that the suspension at one end 
of the series has a low positive potential and at the other end a 
low negative potential. There must then be a place in between 
where there is no potential, even though there is a very large 


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Qa -|3 -1.0 mn 
vine =15 
Agglutination C.to Ht 
oe a 
—-—=No agglutination 
-2.5 





10-6 \0-s 10-4 10-5 lo7e 107! 1.0 
Salt Concentration, equivalents per liter 


Fig. 5.—Effect of salt concentration on the potential and agglutination of 
suspension of B. typhosus. 


error in the method. Nevertheless, no agglutination occurs in 
any tube in the presence of concentrated sodium chloride. 
This is shown in Fig. 6. The salt evidently acts as though it 
prevented the particles from sticking together, even though 
there is no force to hold them apart. It occurred to the writer 
that it might be possible to measure this sticking or cohesive 
force by determining the force required to separate two films of 
the suspension. This turned out to be the case. The measure- 
ment was made by coating two pieces of glass with a thick smear 


COLLOIDAL SUSPENSIONS 87 


of the suspension. The glass was then warmed slightly in order 
to cause the particles to adhere to it, and the two films were 
then allowed to rest together in the solution to be studied. The 
force required to tear the films apart was then determined by a 
torsion balance. The measurement is rough and the conditions 
very different from those existing in the original suspension, but 
nevertheless the results are surprisingly reproducible and show a 
very marked effect of concentrated salt solutions on this cohesive 


Potential in millivolts 
pL per sec. 





me eer rh Ga. 8. 9. 10 
Concentration of HCIx10* 


Fic. 6.—Effect of increasing NaCl concentration on the acid agglutination of 
B. typhosus. ; 


force. Furthermore, the effect is in the range of salt concentra- 
tions where the potential measurements fail to predict the 
agglutination. The results of some of these experiments are 
shown in Fig. 7. It will be noted that the only solution which 
shows a second rise in this value is hydrochloric acid and this 
agrees with a second agglutination zone in this acid (cf. Fig. 5). 
It is evident then that so-called irregular series are not always 
due solely to the potential-changes.*’ It may be noted that in 

37 In saturated salt solutions there is again an agglutination effect (cf. 
Porgss, O.: Centralbl. Bact. Orig., 40 (1905), 133). This is probably a true 
solubility effect similar to that found by Loeb in the case of gelatin-coated 
particles to be discussed later. 


88 COLLOIDAL BEHAVIOR 


the case of so-called autoagglutinable bacteria this effect of strong 
salt solutions does not appear. ‘The bacteria, therefore, aggluti- 
nate in salt solutions of more than about 0.01 Nn without any 
immune serum.*® The difference between diffuse and auto- 
agglutinable strains of these bacteria is, therefore, the same as 
that between sensitized and unsensitized B. typhosus. The 
agglutinable strains and sensitized B. typhosus agglutinate when- 
ever the potential is reduced below the critical value, whereas 


l20p 





"LCR & \ Set 
‘LLLU KN 
HET NSN 


Mg.required to separate 2cm* surface 


Salt concentration , equivalents per liter 


Fig. 7.—Effect of salt concentration on the ‘cohesive force.” 


in the case of the diffuse strains (or B. typhosus alone) the addition 
of concentrated salt solutions prevents agglutination by lowering 
the cohesive force. 

It was stated above that the addition of immune serum to a 
suspension of bacteria caused them to act like collodion or oil 
particles, 7.e., they agglutinate whenever the p.p. is below the 
critical value. It might be expected, then, that the addition of 
serum to bacteria whose potential was already below the critical 

38 Personal communication from Dr. J. Shibley. 


COLLOIDAL SUSPENSIONS 89 


value would cause agglutination without any change in the 
potential, but an increase in the cohesive force. This is the 
result obtained, as is shown in Fig. 8. The figure also shows that 


Cohesion in mg. 





0.00016 0.0008 0.004 0.02 
Concentration of serum 


Cohesion in mg. 





Concentration n NaCl 


Fig. 8.—Upper Curve. Effect of immune serum on cohesive force in 1.0 
mM NaCl. Lower Curve. Effect of salt concentration on cohesive force of 
sensitized and unsensitized smears. 


the cohesive force of a smear treated with immune serum is not 
affected by the salt concentration. 

Under the ordinary conditions of bacteriological agglutination 
in 0.75 m salt, therefore, immune serum causes agglutination by 


90 COLLOIDAL BEHAVIOR 


increasing the cohesive force between the organisms and not by 
affecting the potential. At the same time the antibody in the 
serum combines with the bacteria and probably forms a surface 
film. In this respect immune serum differs qualitatively from 
normal serum in that normal serum has no effect on the cohesive 
force and, hence, will not agglutinate bacteria in salt solution. 
It is possible, however, to arrange conditions so that agglutination 
is caused by a change in potential. This is the case if the sus- 
pension is near its acid agglutination point. Under these condi- 
tions both normal and immune serum will cause agglutination. 
It seems probable, therefore, that the reaction is due to normal 
serum proteins rather than to the specific antibody. 

This effect of immune serum on the cohesive force has not been 
noted, as far as the writer is aware, in connection with any other 
substance. The effect of egg albumin on mastic*® is, perhaps, 
similar. The reverse effect, however, is well known and is the 
mechanism of the protective colloid action. It was noted by 
Faraday that the addition of ‘‘jelly”’ to colloidal gold solutions 
prevented their precipitation by electrolytes. Theaction wasthen 
studied by Meyer and Lottermoser* who stated that ‘‘on the 
addition of very stable colloids, such as albumin, gelatin, agar, 
or gum arabic, to silver sols no precipitation is caused by electro- 
lytes until this stable colloid is gelatinized (precipitated). The 
less stable silver sol is thus protected against the electrolyte by 
the more stable colloid. It becomes more like the latter in its 
behavior.” Zsigmondy*! defined the protective value of a solution 
as the gold number, 7.e., the weight in milligrams of a substance 
which just fails to prevent the change in color of 10 cc. of a 
standard gold solution on the addition of 1 ce. of 10 per cent salt. 
The particles in the presence of the protective colloid, therefore, 
act just as do the bacteria mentioned above, i.e., the potential 
may be reduced to zero by an electrolyte and still no agglutination 
occurs, since there is no attractive force. If the concentration of 
electrolyte is still further increased until the protective colloid 
itself precipitates, the suspension agglutinates. This effect is 


8 MicHagE is, L. and Rona, P.: Biochem. Z., 2 (1906), 219. 

40 Cf. TayLor, W. W.: ‘Chemistry of Colloids,” 3rd ed., New York. 
1915, p. 129. 

41 ZsiGMOnpDy, R.: Z. anal. Chem., 40 (1902), 697, 


ot 


COLLOIDAL SUSPENSIONS 


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9; o% VA0qe 10 34 I w< Te ox Cm re es 0O'? 
% if w< ix a< race Zz fz o's 
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uoTyN[O, | sepryVd | WoTyNjog | sepoyseg | uornyog | sapyswg | uornyjog | seporyeg | UOIyNjo, | sepotyeg 
*(NO)29q'®N | FOS*8N | eT | 2198O | [ORN Hd 
Weg 





Hd SOorldvA LV SNOILOTIOQ NILVTAY) XO 
SHIOILUVG NOIGOTION GALVOO-NILVIE‘) GALVLIdIOGUg OL GHUINday LIVG dO SNOILVULNGONOS) TVYNINIJ[—z; JIT PIav L, 


' 


92 COLLOIDAL BEHAVIOR 


clearly shown by Loeb’s experiments with collodion particles 
treated with gelatin (Table III). As was stated above, the 
precipitation of collodion particles alone is determined solely by 
the p.p. As Loeb pointed out, this precipitation of gelatin- 
coated collodion particles by high concentrations of salts is 
probably a phenomenon of pure solubility analogous to the 
salting out of proteins from solution (cf. below). It may be 
accounted for by the assumption that the salt reduces the attrac- 
tion between the surface molecules of the particles and the 
solvent. * 

Meyer and Lottermoser’s statement that the suspension acquires 
the properties of the added substance has been confirmed, in 
general, by all subsequent investigators. There is little doubt 
that this is due to the formation of a surface. film. It was 
noted by Bredig* that molds grow on the surface of gold particles 
which had been treated with gelatin. Arkwright4® found that a 
suspension of B. colz in water containing soluble substances from 
B. typhosus was agglutinated by anti B. typhosus serum. This is 
a very sensitive test, since the action of immune serum is strictly 
specific. It has also been noted by a number of workers that the 
isoelectric point of a suspension changes to that of the added 
substance. *® 

It must be noted, however, as pointed out by Loeb, that the 
behavior of the coated particle may differ in some respects from 
that of the protective colloid. Collodion particles, for instance, 
coated with native egg albumin precipitated at the isoelectric 
point of egg albumin, whereas egg albumin itself does not precipi- 
tate under these conditions. Loeb suggests that the protein 
is denatured by the formation of the film, as has been shown to be 
the case at the air-liquid surface. It follows from the above 
mechanism of the action of protective colloids that two or three 
characteristics are necessary: (1) the protecting substance must: 


43 Lorn, J.: Loc. cit. 

44 Brepic, G.: “Anorganische Fermente,” Leipzig, 1901, p. 15. 

4 ARKWRIGHT, J. A.: J. Hygiene, 14 (1914), 261. 

 ArKwricut, J. A.: Loc. cit.; Coutter, C. B.: J. Gen. Physiol., 4 (1921- 
22), 403; Loxs, J.: J. Gen. Physiol., 5 (1922-23), 109, and several earlier 
papers; Norturop, J. H. and Dr Kruir, P. H.: J. Gen. Physiol., 4 (1921-22), 
655. 


COLLOIDAL SUSPENSIONS 93 


form a surface film on the particles; (2) there must be no attrac- 
tive or cohesive force between two such surface films; or (3) the 
resulting particle must be highly charged. 


NATURE OF THE COHESIVE FORCE 


The theory of the stability of colloidal suspensions outlined 
above predicts that coagulation will occur whenever the cohesive 
force is greater than the repulsive force. ‘There is, in addition, 
very good evidence that the velocity of migration in an electric 
field is a measure of the repulsive force. There is every reason to 
believe that this rate of migration is, in turn, proportional to the 
potential difference between the surface of the particle and 
the surrounding liquid, and that the repulsive force is due to 
the mutual repulsion of this electric charge. The nature of the 
attractive force, however, is much less certain. The majority 
of writers on the subject state that it is a ‘‘surface tension”’ or 
‘“‘capillary”’ effect and Billitzer*” assumed that the change in the 
potential was simply a measure of the change in the surface ten- 
sion, the latter approaching a maximum as the Pp.D. approached a 
minimum. ‘The same viewisexpressed by Michaelis.4® v.Smolu- 
chowski*? also assumes that the attractive force is increased as the 
potential decreases. Freundlich®® assumes the attractive force 
to be constant. If the writer’s measurements of this force are 
significant, there is no doubt that it does not vary in any way 
with the potential. There is no valency effect and the effects of 
salt are all in concentrations so high that the potential is very 
low or absent. It must be noted that in Powis’ experiments with 
oil drops there was no coalescence of the drops. There is no 
change in the oil-water surface, therefore, and hence it cannot be 
the oil-water surface tension which draws the drops together. 
Whatever force is active must reside in the surface film surround- 
ing the drop, since it is these films that coalesce. There is no 
evidence of molecular contact of the particles. 


47 BILuITzER, J.: Z. physik. Chem., 51 (1905), 128. 

48 Micnaruis, L.: ‘Die Wasserstoffionenkonzentration,” 1st ed., 1914, 
p. 49. 

49 vy, SMoLucHowskI, M.: Physik. Z., 17 (1916), 557, 583. 

50 FREUNDLICH, H.: Kolloid-Z., 23 (1918), 163. 


94 COLLOIDAL BEHAVIOR 


THE VELOCITY OF COAGULATION 


The changes in a coagulating suspension or sol take place 
relatively slowly and complete coagulation may require several 
hours. The change in the potential on the addition of the 
electrolyte, however, is almost instantaneous, at least in the 
suspensions of bacteria studied by the writer and also in Powis’ 
experiments. The time element, therefore, consists in the time 
required for two or more particles to meet and stick together. 
Smoluchowski* has been able to derive a formula for the rate of 
this reaction based on the probability of collision of the particles. 
He assumes, as did Zsigmondy, that the particles are uncharged 
and that the collisions are inelastic, z.e., every collision between 
two particles results in the formation of an aggregate of two 
particles. This leads to the equation 


lj 1 1 
k = -(5, - a) = 4xDr 

in which V, is the number of particles present at the beginning. 
xv the number present at time t, D the diffusion coefficient, and r 
the distance between the particles at which they are attracted. 
It is assumed, further, that this is not much greater than twice 
the radius of the particles. It may be noted that the formula is 
the same as that for a bimolecular reaction, with the exception 
that all collisions are considered as leading to combination, 
whereas, as is known, such an assumption will not hold with 
respect to chemical reactions. The theory predicts the experi- 
mental results accurately, as was found by Westgren and Reit- 
stotter and by Kruyt and van Arkel.® 

In the above derivation it was assumed that the particles 
possessed no repulsive force. This is the condition at the 
isoelectric point. Experimentally, however, it is known that 
agglutination occurs when the particles are slightly charged. 
Under these conditions it might be supposed that only those 
particles having sufficient kinetic energy would be able to approach 
each other within the “attraction sphere.” The coagulation 
would be, therefore, slower but eventually would become com- . 


*1 vy. SMoLucHOowsKI, M.: Physik. Z., 17 (1916), 557, 583. 
°? FREUNDLICH, H.: “Kapillarchemie,’”’ 2nd ed., Leipzig, 1922, p. 596. 


COLLOIDAL SUSPENSIONS 95 


plete, as is the case. Freundlich** has taken this effect into 
account and derived a complete formula which correctly predicts 
the course of the coagulation. 

In certain cases—‘‘slow coagulation’’—the rate curve resem- 
bles that of an autocatalytic reaction.*4 According to v. Smolu- 
chowski, this isa secondary phenomenon. It seems possible that 
this effect is due to a slow change in the potential. It was found 
by Powis, for instance, that there was a very rapid change in the 
potential immediately on the addition of the electrolyte.. This 
was followed, however, by a second slow decrease extending 
over a period of days. If the potential immediately after the 
addition of the electrolyte were slightly above the critical value, 
it is readily seen that the flocculation might be caused by the 
second slow potential change. It is just in these cases that the 
peculiarity manifests itself. When the potential is reduced at 
once to zero, ordinary rapid coagulation occurs. 


THe Errect OF THE NATURE AND VALENCE OF THE ION ON THE 
PRECIPITATION OF SUSPENSIONS 


It has been noted above that precipitation of most suspensions 
is caused by the addition of the proper amount of electrolyte and 
that this effect is due to the change in either the cohesive force 
or the potential. The amounts of the various ions necessary to 
cause these effects, however, are very different. Two general 
statements may be made which agree fairly well with practically 
all experimental results: (1) The effect on the potential and stabil- 
ity is primarily due to the ion of the opposite charge to that of the 
suspension. (2) The concentration of electrolyte needed to cause 
precipitation decreases rapidly as the valence increases. This 
increase is approximately as the square of the valence—the 
Hardy-Schulze rule. Neither of these statements, however, is 
strictly true. The potential is often increased by an ion of the 
same charge, for instance, the addition of acid to a suspension of 
denatured protein which is already slightly positive, or the addi- 
tion of an excess of trivalent ions. The precipitating effect, 
however, is due to the oppositely charged ion, as stated above. 


*3 FREUNDLICH, H.: Kolloid-Z., 23 (1918), 163. 
54 LorreRMoSER, A.: Kolloid-Z., 15 (1914), 145. 


96 COLLOIDAL BEHAVIOR 


The effect is not purely a question of valence. Hydrogen and 
hydroxyl ions differ as a rule in their action from the other 
monovalent ions and behave more like the trivalent ions. The 
ions of the heavy metals behave differently from those of the 
alkalies and alkaline earths. The problem is complicated by 
the fact that it is difficult in many cases to vary the concentra- 
tion of the ion in question without at the same time varying the 
hydrogen ion. 

It was found by Whitney and Ober*® that the amounts of the 
various ions required to cause complete precipitation of arsenic 
sulfide sols were chemically equivalent. This was determined 
by analyzing the precipitate. Equivalent amounts of the 
precipitating ion were bound. The same result was obtained 
by Duclaux.*® 

There is no simple relation, however, between the concentra- 
tion of the added electrolyte and the amount combined with the 
particles. The results are best expressed by the exponential 
adsorption equation. As Freundlich has pointed out, therefore, 
it is not possible to determine how much electrolyte is combined 
with the precipitate from the total concentration of electrolyte.*? 
The theory already mentioned, that the p.p. of the particles 
is due to a Donnan equilibrium, will account qualitatively, at 
least, for the experiments, in that it predicts a valency effect and 
a difference in the behavior of the electrolyte, depending on 
whether it combines with the particle or merely affects the relative 
concentration of ions inside and outside the particle. MceTag- 
gart’s experiments with air bubbles again seem to require a separate 
explanation. It cannot be supposed that the air adsorbs the ion 
or combines with it in any way, yet the potential of these air 
bubbles was reversed by trivalent ions, as are most other particles. 
It is evident that much more quantitative work on the combina- 
tion of ions with finely divided matter is necessary before any 
general theory can be stated. 

The mechanism whereby the potential is affected by proteins, 
etc., is still more uncertain than in the case of electrolytes. There 


°° Wuitney, W. R. and OsEr, J. E.: J. Am. Chem. Soc., 23 (1901), 842. 

°° Cf. Taytor, W. W.: “Chemistry of Colloids,” 3rd ed., New York 
1915, p. 109. 

*’ FREUNDLICH, H.: ‘‘Kapillarchemie,’”’ 2nd ed., Leipzig, 1922, p. 581. 


COLLOIDAL SUSPENSIONS 97 


is every reason to believe that in this case it is a surface film 
formation, but whether this film is of oriented molecules, as in 
Langmuir’s experiments,** or simply a concentration in the sur- 
face layer or some type of physical combination, is uncertain. 
It has been suggested that the combination is due to the attrac- 
tion of the opposite charges, but this cannot be the decisive factor, 
since the addition of ‘a positive suspension to another positive 
suspension can result in increasing the positive charge. It is 
known, on the other hand, that positive particles are retained by 
negative filters, whereas negative ones are allowed to pass.*? 

It may be noted, however, in cataphoresis experiments with 
bacteria®® that the glass cell becomes more or less coated with 
bacteria which adhere firmly even in solutions in which both the 
glass and the bacteria are negative. It is possible that a quanti- 
tative effect could be noted if the glass and bacteria were given 
opposite charge, as in acid solution. 


PRECIPITATION OF NATIVE PROTEINS 


The salting out of proteins has often been considered as analo- 
gous to the precipitation of a colloidal suspension. It has been 
shown by Sorensen in the case of egg albumin that this is not the 
case but that the phenomenon is one of ordinary solubility.*! 
The same is true of the solution of casein in alkali.*? In the 
precipitation of casein from solution by the addition of acid, both 
factors enter. The formation of insoluble isoelectric casein is 
regulated by the pH, as predicted by the solubility product. 
These particles, however, then become protected by the native 
protein in the solution and so do not precipitate except in a very 
narrow zone at the isoelectric point where the solubility of the 
native protein is very small. 


88 LANGMuIR, I.: J. Am. Chem. Soc., 38 (1916), 1145. 

5° TayLor, W. W.: “Chemistry of Colloids,” 3rd ed., New York, 1915, 
p. 59. 

60 NortHROP, J. H.: J. Gen. Physiol., 4 (1921-22), 633. 

61 SORENSEN, S. P. L.: Z. physiol. Chem., 103 (1918), 211. 

62 CoHEN, E. J. and Henpry, J. L.: J. Gen. Physiol., 5 (1922-23), 521. 


CHAPTER IV 


THE COLLOIDAL BEHAVIOR OF THE BODY FLUIDS 
By 


Donaup D. VAN SLYKE 


Of the substances customarily classed as “colloids,” the proteins 
contribute the greatest part to the body material. Their part in 
vital processes may be no more important than that of the lipoidal 
colloids. The outstanding researches of Hamburger, Donnan, 
and Loeb have, however, given us the basis to measure and study 
some of the phenomena dependent on two colloidal properties 
possessed by many proteins, low osmotic pressure and inability to 
pass through membranes, and on the ability which the proteins 
possess, by virtue of their amino acid structure, to combine with 
acids and bases. The study of the behavior of body colloids 
other than proteins has not progressed sufficiently far to provide 
material for definite conclusions. For this reason we limit our 
discussion to the proteins, with full realization that the resulting 
presentation must be incomplete. 

The osmotic and amphoteric properties of the proteins in water 
solution have been studied with a gradually increasing under- 
standing for the past three decades. It was, however, only after 
Donnan’s studies, of the behavior of diffusible and non-diffusible 
ions in solutions separated by membranes, became available 
that the peculiar rdle which the proteins play in controlling the 
distribution of other electrolytes and of water in the body could be 
studied in a rational manner. The application of Donnan’s 
theory to protein solutions in vitro was carried out by Procter 
and Wilson and particularly by Loeb and his collaborators. The 
study has been recently extended to the animal organism by 
Warburg and by the writer, working with Wu and McLean. 
The studies thus far have been limited to the more accessible 
body fluids, blood and transudates. It is probable that in the 

98 


THE COLLOIDAL BEHAVIOR OF THE BODY FLUIDS 99 


near future data will become available on the less accessible parts 
of the body, such as the muscles, where the work of Meyerhof 
indicates the buffer character of the proteins is an important 
factor in the cycle of muscular activity. At present, however, 
the discussion is practically limited, by the available material, 
to the blood and transudates. The following treatment of the 
problem is essentially that developed by Van Slyke, Wu, and 
McLean. We shall consider, first, the known facts concerning 
the relationships of electrolytes and proteins, then, their inter- 
pretation according to the physico-chemical laws of solutions, 
and, finally, certain deductions from the conclusions drawn. 


OBSERVED Facts CONCERNING THE PROTEINS AND ELECTROLYTES 
OF THE BLOOD 


The facts concerning the blood, which may be accepted on the 
basis of data already in the literature, are the following: 

1. The osmotic pressure of the fluid within the cells appears to 

equal that of the serum outside. The disc shape of the erythro- 
cyte indicates the absence of internal pressure. The latter 
would force the cell to assume a globoid shape, as it tends to do in 
hypotonic solutions. 
_ 2. In both cells and serum the positive charges of the alkali 
cations are balanced in part by negatively charged, non-diffusible 
protein anions, and in part by diffusible anions, of which Cl’ and 
HCO,’ constitute the greater part. 

3. All the non-protein ions normally present in amounts con- 
tributing significantly to the total osmotic pressure are monova- 
lent. These are Kt, Nat, Cl’, and HCO;’. Mgt+, Catt, S04”, 
and HPO,” are present in relatively such small amounts that in 
an approximation of conditions controlling the total osmotic 
pressure they may be neglected (Kramer and Tisdall, Zucker 
and Gutman). 

4, Of the cell and serum proteins, only hemoglobin exerts a 
significant part of the total osmotic pressure. 

That the electrolyte molecules and ions constitute nearly all of 
the osmotically active substances present is shown by the corre- 
-spondence between the lowering of the vapor tension observed 
(Neuhausen) and the lowering attributable to the electrolytes 


100 COLLOIDAL BEHAVIOR 


present. The chief non-electrolyte crystalloids, glucose and 
urea, themselves diffuse through the cell membranes and, 
therefore, cannot influence the water distribution. They are, 
furthermore, present in relatively small amounts, about 5 and 
3 millimoles respectively out of a total osmolar concentration! 
of 300. 

Of the proteins, it appears that hemoglobin is the only one that 
exerts more than 1 per cent of the total osmotic pressure in either 
cells or serum. Hiifner and Gansser found that electrolyte-free 
ox and horse hemoglobin exert the osmotic pressures calculated 
on the assumption that 1 molecule of oxygen combines at atmos- 
pheric pressure with 1 molecule of hemoglobin, and we have based 
our calculations of the osmotic effect of hemoglobin on these 
results. It will be seen from the tables that hemoglobin is 
estimated to exert about 10 per cent of the total osmolar concen- 
tration of the cells.? 

The serum proteins, according to Starling, exert 30 to 40 mm. of 
pressure, or less than 1 per cent of the total (estimated at 0.300 
ss x 22.4 X 760 = 5,800 mm.). Presumably, the cell pro- 
teins other than hemoglobin exert still less pressure, because 
of their small amount. It appears, therefore, that in calculating 
the total osmotic effects of the blood the serum proteins and the 
cell proteins other than hemoglobin may be neglected. 

5. The cell membranes are permeable to water, carbon dioxide, 
to the inorganic anions, and to either H+ or OH’, or both. 

In water solutions the same [H+] would result, whether the 
membrane is permeable to [Ht], or [OH’], or to both. The 
concentration of either ion varies inversely as that of the other, 


according to the equation [H+] = OWT Any factor which fixes 


1 We have adopted the convenient term ‘“osmolar” concentration intro- 
duced by Warburg, to indicate the total concentration of osmotically active 
ions and molecules. 

* Adair, in a personal communication, states that measurements that 
he has made in L. J. Henderson’s laboratory indicate in dilute solutions a 
much higher molecular weight for hemoglobin than that found by Hifner 
and Gansser, but that in concentrations approaching those in the cells 
other forces augment the osmotic power of the hemoglobin to about that 
which corresponds to Hiifner and Gansser’s measurements. 


THE COLLOIDAL BEHAVIOR OF THE BODY FLUIDS 101 


{H+] therefore fixes [OH’], and vice versa, so that it is impossible, 
and likewise, for our present purposes, immaterial, to tell whether 
the membrane is permeable for [H+], [OH’], or both. 

6. The cell membranes are impermeable to the proteins, ionized 
or not, and to K and Na (Giirber, Doisy and Eaton). 

7. The physiological pH ranges of the cells and serum are on 
the alkaline side of the isoelectric points of the cell and serum 
proteins (Michaelis). Consequently, in the body the blood 
proteins combine with alkalies, but not with acids in amounts 
significant for the purposes of this paper. 

8. The amounts of alkali bound by the cell and serum proteins 
increase in approximately a linear manner with increasing pH over 
the physiological range. The rate of change in protein-bound 
alkali per unit change in pH is several times as great in the cell 
fluid as in the serum. 

9. At physiological pH ranges, reduced hemoglobin binds less 
alkali (0.5 to 0.7 equivalent less per molecule of hemoglobin) than 
does oxygenated hemoglobin. 


Tur SoLuTIon LAws INVOLVED IN BLoop RELATIONSHIPS 


In combining the above facts to form an inclusive quantitative 
expression of the phenomena of electrolyte and water distribution, 
we have assumed for the blood the validity of the following 
physico-chemical laws: 

1. At and near the neutral point all strong alkalies in quanti- 
tatively significant amounts are in the form of salts. At blood 
reaction, therefore, the total base B may be represented as BP + 
BA, where BP represents the alkali protein salts, in equivalents of 
monovalent alkali, and BA the salts formed by the alkali with 
other negative radicles, chiefly Cl’ and HCO’. 

2. The law of Donnan governing the influence of non-per- 
-meating ions on the distribution of permeating ions on the two 
sides of a membrane holds for the membranes of the blood cells. 

3. The osmotic activity of each solute is proportional to the 


ratio = of gram molecules of solute to gram molecules of water. 


The presence of the serum proteins, according to the vapor ten- 
sion determinations of Neuhausen, does not affect the validity of 


102 COLLOIDAL BEHAVIOR 


this ratio as the governing factor of osmotic activity, and data of 
Van Slyke, Wu, and McLean show that the cell proteins likewise 
fail to affect it. With dilute water solutions it makes relatively 


little difference whether the ratio a or is taken as a 
2 


volume 
measure or osmotic activity. In the blood cells, however, where 
the water constitutes only 60 to 65 per cent of the total contents, 
the difference is of importance. 


Bjerrum (quoted by Warburg) considers the ratio cae to 


be a better indicator of osmotic activity in concentrated solutions 


n 


than the ratio Rs In blood, however, 7 is less than 0.01 as great 


as N, so that within the limits of experimental error it is immate- 
rial which of these two ratios we use. Consequently, we shall 


employ the simpler, a 


For our calculations, in place of using gram molecules of water 
as the unit of the denominator, we have used kilos of water, in 
order to express the results in terms not unnecessarily removed 
from the familiar gram molecules per liter unit. 

The relationships expressed above under (1), (2), and (3) may be 
expressed in certain basic equations, which, when combined, yield 
a practical and simple expression indicating the quantitative 
relationships of the factors discussed. 

1. For the approximate neutrality of the blood reaction, 
[OH’] and [H+] being negligible compared with the other ions, 
we have 





[B], = [BA], + [BP], (1) 

[Bic = [BA]. = [BP]. (2) 

The brackets are used to indicate concentrations in terms of the 

ratio coe The subscripts , and , indicate serum and cells 
water 


respectively. B, BA, and BP have the significance used under (1) 
in the preceding discussion. (For simplicity we indicate all the 
alkali bound to non-diffusible acids as BP, although a small 
part may be bound by substances other than proteins, such as 
conjugated phosphates. ) 


THE COLLOIDAL BEHAVIOR OF THE BODY FLUIDS 103 


2. For conformity with Donnan’s law the diffusible monovalent 
ions have the following relationships: 

H+}, _ (C', _ (HCO. [OH [A _, 
ae |, ) (HCO;  [OH’), — [A]. 

A’, and A’, represent the sums of all the monovalent anions. 
For convenience we shall use the factor r to express the ratio 
indicated. 

[B]. and [B], do not appear in equation (8), for they are not 
diffusible. If they were, in addition to the conditions defined in 
equations (1), (2), (3), and (4), we should be required to make our 


final equation conform to the condition that na = ey ae 




















and the results would be altogether different. 
3. For osmotic equality the ratio of osmotically active mole- 
cules and ions to water is the same in serum and cells. 
Fo = oe ot UML = 2M. (4) 
In equation (4), M, and M, represent the osmotically active 
ions and molecules, and =[M], and =[M], the sums of their total 
solute 
water 





concentrations, in terms of the ratio, in serum and cells, 


respectively. 
As alternative forms of equation (4), we may write, if we assume 
complete dissociation of the electrolytes: 


[B]. + [Cl]. + [HCOs3], = [B]. + [Cl]. + [HCO3]. + [Hb]. (5) 
2{BA], + [BP]. = 2[BA]. + [BP]. + [Hb]. (6) 
2(B], — [BP]. = 2[B]. — [BP]. + [Hb]. (7) 

Equation (5) merely expresses the sum of the total ions in 
serum, and of ions plus hemoglobin molecules in the cells, complete 
dissociation being assumed, and likewise a balancing, in serum 
and cells, respectively, of the small amount (not over 5 per cent of 
the total) of osmotically active substances (PO,’’, SO4’’, etc.) not 
represented in the equation. Hb is expressed in units of mols of 
Oz capacity. 

In equation (6) the total osmolar concentration is represented 
as twice the molecular concentration of the salts with monovalent 
ions and cations (since each dissociates into two ions) plus once 
the concentration of base in the form of protein salt, since the 


104 COLLOIDAL BEHAVIOR 


osmotic effect of BP is due to the alkali cation. In the cells we 
add also the osmotic effect of the hemoglobin, which is assumed 
to be the same regardless of the ionic charge of the hemoglobin 
molecules. 

Equation (7) is derived from equation (6) by substituting [B] — 
[BP] for [BA], according to equations (1) and (2). 

As stated above, equations (5), (6), and (7) are theoretically 
accurate if the electrolytes are completely dissociated into osmot- 
ically active ions. The observed osmotic behavior of alkali salts 
in general does not justify the assumption that dissociation is 
complete, and Neuhausen and Marshall from electrometric 
measurements have estimated that the sodium salts in blood 
serum are 83 per cent dissociated. However, if we assume, not 
complete dissociation, but equal dissociation of the salts in cells 
amd serum, respectively, the relationships expressed in equations 
(5), (6), and (7) still hold, not exactly, but so nearly that the 
deviations may be neglected for present purposes. 

The theoretical inexactness of equations (5), (6), and (7) when 
y, the degree of dissociation, is less than 1, even though y is 
equal on both sides of the membrane, arises as follows. When y 
becomes less than 1, although [Cl], [HCO;], and the part of [B] 
balanced by [Cl] and [HCOg], are all multiplied on both sides of 


1 : ; : 
the equation by the same factor, =, to give their osmotic 


activities, the part of B present as BP is multiplied by a smaller 
factor, y, and the [Hb] by a larger factor, 1. The two deviating 
Ate 

2 
which is their mean; they apply in blood to relatively small parts 
of the total osmotically active solutes; and they partially balance 
their effects, which, to judge from our éxpareeenel results, exceed 
but little our present limits of experimental measurement. 

The basic assumptions made under (1) and (2), and expressed 
in equations (1), (2), and (3), stand on experimental data familiar 





factors, y and 1, however, are not greatly different from 


ratios in cells 





: : lut 
in the literature. The assumption of equal ae 
water 


and serum, expressed under (3) in equation (4), and in equations 
(5), (6), and (7), is supported by the analyses of ae Slyke, Wu, 
and McLean (1923). 


THE COLLOIDAL BEHAVIOR OF THE BODY FLUIDS 105 


ELECTROLYTE DISTRIBUTION 


Dividing equation (6) through by 2[BA], and rearranging it we 
obtain 
[BA]. _ , _ [BP]. + [Hb]. — [BPI, iy 
[BA], 2[BA], : 


We may assume that, whatever the dissociations of the dif- 
ferent salts with the monovalent anions, the salts with identical 
anions are dissociated to nearly the same extent in serum and cells 
so long as the concentrations do not differ greatly. This assump- 
tion appears justified even though the cations in the cell are nearly 
all K, while those in the serum are nearly all Na; for, whether 
conductivity or freezing point data are considered, the differences 
in dissociation found between potassium and sodium salts with 
the same anions at similar concentrations are slight. We may 
then write, with approximate accuracy, 














[BA]. _ [A’]. _ 
[BA[,  [A’]. 
[A’]. 
From equation (3), Taian r. From equation (1), [BA], = 
[B], — [BP],. Substituting, in equation (8), r for eat in the 


left-hand member, and [B], — [BP], for [BA], in the right-hand 
member, we obtain the following equation, showing the approxi- 
mate relationship between the distribution of diffusible cons and the 
amounts of alkali combined with the non-diffusible substances 
(proteins) of the cells and serum. 


_ [H+], _ (Cll, _ [BHCO,). _ , _ (BPI. + [Hb]. ~ [BPl. (9) 
ea cl, ~ (BHCO,). 2([B]. — [BP].) 











We may expect the three ratios in equation (9) to vary from 
equality to each other, and to the r calculated from the right- 
hand member of the equation, in proportion as the y, and, perhaps, 
secondary factors affecting osmotic activity, vary in the cells from 
the like factors in the serum, but we may expect these variations 
in the ratios not to exceed a few per cent of their values. 

The experiments of Van Slyke, Wu, and McLean and of Van 
Slyke, Hastings, Heidelberger, and Neill with horse blood have 


106 - COLLOIDAL BEHAVIOR 


shown the numerical values [BP]. and [BP], may be approxi- 
mately calculated by the empirical formulas (10) and (11). 


[BP], = 0.068[P],(pH, — 4.80) (10) 
[BP]. = 3.35[Hb].(pH. — 6.74) + [Oo].(0.25pH, — 1.18)(11) 


Equation (9) suffices for determining whether results obtained 
with a given blood agree with the quantitative requirements of 
the laws on which these equations are based. Because of the 
variation in water distribution with changing pH and oxygen con- 
tent, however, the concentrations even of the non-diffusible con- 
stituents [Hb]., [P]. and [B], are variable. Consequently, 
equation (9) cannot be used to predict the r curve of a given blood 
with varying pH. However, by combining equation (4) with 
(9), one is obtained in which all the values on the right side are 
functions of values which are constant for a given blood, wz., 
(B)., (B)s, (Hb), and (P).. 

In the remainder of the chapter we shall utilize parentheses to 
indicate units of substance per unit of whole blood, e.g., (H20), = 
kilos .of cell water per kilo of blood, (P), = grams of serum 
protein per kilo of blood, and (B), = millimols of serum base 
per kilo of blood, as contrasted with the bracketed [B],, which 














indicates the ratio pee rao 
In equation (9) we substitute mor, £ rie Gee for 
[Hb]., etc. We thus obtain 
r= 1~ Gro) + 21@)= wep) TERE). Pa 2) 
From equation (4) we have LOY = oat Substituting 


Bap apy. for a (see discussion of equation (7)) 





we get 
CHO} 2(B), — (BP). 
(H:0).  2(B). — (BP). + (Hb) 





(13) 


THE COLLOIDAL BEHAVIOR OF THE BODY FLUIDS 107 


(H20), 














Substituting in equation (12) the value for (H.0) from equa- 
tion (13) we obtain 
— ee el. (HCO,'. ds seen (BP). + (Hb) ns 
lee. | {HCOs'|. 2(B). — (BP), + (Hb) 


(BP). 
2{(B). — (BP).} 





(14) 


If the indiffusible substances, base and proteins, in the cells are 
assumed to maintain constant relations to each other, and the 
indiffusible substances within the serum are assumed to do like- 
wise, it becomes possible to express as functions of (Hb) the other 
three constants, (P),;, (B);, and (B).. Under these circumstances 
(B). is proportional to (Hb), and the serum base and protein, 
(B), and (P),, decrease by amounts proportional to (Hb). Thus, 
from the data in the experiments on normal horse blood by Van 
Slyke, Wu, and McLean, we have with a fairly close degree of 
constancy: 


(B), = 6.0(Hb) (15) 
(B), = 148 — 8.3(Hb) (16) 
(P), = 0.072 — 0.0039(Hb) (17) 


In equations (16) and (17) the first numerical constant in each 
represents the average value at normal pH for serum free from 
cells, and when, therefore, (Hb) = 0. The second constant 
indicates the rates of change in (B), and (P),, respectively, per unit 
of increase in hemoglobin, when the hemoglobin is measured in 
terms of millimols of oxygen capacity per kilo of blood. 

From inspection of equation (14) it is evident that the fraction 
Ba) (BP). oe iby expressing the effects of the cell factors, 
is at a given pH, constant for all bloods, whether of high or low 
hemoglobin content, as long as the ratio of base to hemoglobin in 
the cells remains constant, for then all the terms in both numera- 
tor and denominator vary directly as (Hb) (see equations (15) 
(BP). 
2{(B).— (BP).}’ 
expressing the effect of the serum factors, varies slightly, 
at constant pH, with the hemoglobin content of the blood. 





and (55)). The second fraction of equation (14), 


108 COLLOIDAL BEHAVIOR 


But the variation is so small, and the total effect of this fraction 
on the value of r relatively so little, that the r value is, within 
the limits of experimental determination, independent of the hemo- 
globin content of the blood, even when the latter varies over such 
a wide range as from 3 to 12 millimolar, corresponding to from 
7 to 27 cc. of oxygen capacity per 100 g. of blood. 


a= 
1.0 


as 


=pHs- plc 


~logr 





pHs 


[H+], _ {Cl'le _ [HCOs']. 
[Ht[. [Cl’]s [HCO3']s 
14 for horse blood of average serum and cell composition observed in four experi- 
ments are indicated by the curves. The observed chloride and bicarbonate 


ratios in the experiments of Van Slyke, Wu, and McLean are indicated by the 
marked points. 


Fig. 1.—Values of r = 











calculated by Equation 


Consequently, we may represent the average normal r, pH 
relationship by a single curve, which holds for bloods of varying 
hemoglobin content, if the other non-diffusible constituents 
maintain towards the hemoglobin the relationships indicated by 
equations (15), (16), and (17). The curves obtained by substi- 
tuting in equation (14) the values of (B), and (B), indicated by 
equations (15) and (16) when (Hb) = 9, and the values of (BP), 


THE COLLOIDAL BEHAVIOR OF THE BODY FLUIDS 109 


calculated from the (P), indicated by (17) are given in Fig. 1. 
Since, according to Donnan’s law as expressed in equation (3), 

_ [H*], 
ee, (H*. 





> we may write 


— log r = — log [H+], + log [Ht], (18) 
pH, — pH. 


The values of — log r, therefore, indicate the pH differences 
between the serum and cells. These values we have plotted in 
the curves indicated in Fig. 1. 

From data of quite a different nature, obtained on whole 
blood, serum, and hemolyzed blood and cells, and based in part 
on electrometric determinations, Warburg has estimated the 
pH, — pH, values in horse blood at varying pH,. Comparison 
shows that our — log r curve is parallel throughout and nearly 
identical with the curve indicating the maximum pH, — pH, 
values estimated by Warburg.® 


WATER DISTRIBUTION AND CELL VOLUME 


The distribution of water between cells and serum, and the 
resulting volume effects, may be predicted from the pH and the 
degree of oxygenation of the blood if the amounts of non-diffusible 
substance, v2z., base and protein, in the cells and serum, respec- 
tively, are known, and if the law of equality of osmolar concen- 
trations expressed in equation (4) is valid for blood. 

From the general statement expressed in equation (4) we have 


CBO are CNL) peels 1 SCM), 
(H:0), 2(M), 2(M), + 2(M). 


moje DMD. SM), 
(H.0), 2(M),~ SQM), + ZOD. 


where (H.2O),, (H20)., and (H2O), represent the fractions of a 
kilo of water present, respectively, in the serum, cells, and whole 
of a kilo of blood, 2(M),, 2(M)., and 2(M),, the total osmolar 
units (millimols) in the serum, cells, and whole of a kilo of 
blood. 


3See WARBURG, p. 230, curve I, Fig. 11. 





(19) 














(20) 


110 COLLOIDAL BEHAVIOR 


Substituting for =(M), and =(M), their values as in equation 
(7), and replacing (B), + (B), by (B), in the resulting equations, 
we obtain 

(H.2O), * 2(B), — (BP), 

(H20), 2(B), — (BP), — (BP),+ (Hb) 
(H.0), | 2(B), = (BP). ae 
(H:0),  2(B), — (BP), — (BP). + (Hb) 

Multiplying equations (21) and (22) through by (H.O), we 
obtain 


(730), = (He), x 


(21) 








(22) 








2(B). — (BP). 
2(B)» — (BP). — (BP). + (Hb) 


2(B)- — (BP). + (Hb) 
2(B), — (BP), — (BP). + (Hb) 


The above equations, the validity of which has been tested 
experimentally, enable one to predict the amounts of cell and 
serum water per unit weight of blood in terms which are either 
determinable constants (B)., (B)., and (Hb), for a given blood, 
or which may be calculated from such constants, viz., (P). 
and (Hb), and from the pH and oxygen content. The effects 
of pH and of oxygen saturation may be introduced as in equation 
(Ly, 

Within limits, the increase of volume produced by adding a 
solute to a solvent approximates a linear function of the amount 
of solute added, and in both cells and serum nearly all the variable 
solute is protein. We may, therefore, with approximate accuracy 
write 





(23) 





(H20); = (H30), 





(24) 


. = Gy{(H20), + m(Hb)} (25) 
. = G{ (H20), +n (P)s} (26) 


where Gz is the specific gravity of the blood, with water at the 
same temperature as unity; m and n represent the volumes 
occupied in solution by a unit of cell and serum protein, respec- 
tively. When Hb and P, are expressed in gram units, m and n 
both have values somewhat less than 1, since a gram of protein 
occupies somewhat less than 1 cc. volume. For horse blood we 
have found m = 0.90 and n = 0.85 when Hb and P, are expressed 


THE COLLOIDAL BEHAVIOR OF THE BODY FLUIDS 111 


as grams of protein. When Hb is expressed in millimols of 
Os capacity, m = 0.90 < 0.0167 = 0.015. 

Introducing the numerical values of m and n in equations (25) 
and (26) we obtain 


ee = G.{ (H20). + 0.015(Hb) } (27) 
b ; 
(Hb) being expressed in millimols of oxygen capacity per kilo 
of blood, and 

us 

rie Go{ (H20). + 0.85(P).} (28) 
(P), being expressed as grams of serum protein per gram of 
blood (not, for this equation, as grams per kilo of blood). 

The value of G, and (H2O),, constant for a given blood, may be 

estimated for normal horse blood as 


Gp, = 1.027 + 0.0037 (Hb) (29) 


and 
(H2O), = 0.914 — 0.015 (Hb) (30) 


The numerical constants in equations (29) and (30) are obtained 
as described in connection with equations (16) and (17), the first 
constant in each equation representing the G, or (H2O) value for 
normal serum, the second constant representing the change per 
unit increase in (Hb). 1 

The agreement of the (H2O), and (H2O), values calculated at 
varying pH by equations (23) and (24) with observed values of 
Van Slyke, Wu, and McLean is indicated in Fig. 2. 

Warburg has estimated the changes in cell volume with 
varying pH by measuring the oxygen capacity of the cells. The 
number of his determinations is sufficiently large to permit the 
plotting of an average curve by means of which the errors, which 
appear inherent in any method thus far used in estimating the 
small percentage changes in cell volume involved, are to a con- 
siderable extent neutralized. Warburg expresses his results in 
volume of cells at varying pH, compared with the volume at pH, 
6.5. In Table I we have calculated for a blood, of the average 
hemoglobin content ((Hb) = 11.3 millimolar) of the bloods used 
by Warburg, the change in cell volume as estimated by equa- 
tion (27). We have used as the unit of comparison the volume at 


112 COLLOIDAL BEHAVIOR 


pH, 6.8 instead of pH, 6.5, for the reason that both our experi- 
mental data and Warburg’s are less complete and appear less 
certain below pH 6.8 than above it. 





0,68 
0.64 

0.63 

Be = 
eee 

ress 

m+ te lo.56 

sis ee 
aie G S 0.55 ‘3 
3 Geen... - 
‘8 0.54 Gy 
WO aks 
2 | a 
: ee 
oe Lee 
: ee ay 0.60:= 
: ese | Sai 
zs al n= SO 0.59 = 
E fo | ee ae 
aS 

5 Bee 
0.60 

0.59 

058 

057 





pHs 
Fig. 2.—Comparison of observed and calculated water distribution. Cell 
and serum water contents calculated by Equations 23 and 24 are indicated by 
the curves. Water contents observed by the gravimetric and specific gravity 
methods are indicated by the marked points. (From Van Slyke, Wu, and McLean). 


The changes observed by Warburg agree with those calculated 
by equation (27) within the limit of experimental error, as do the 
changes in Fig. 2, except in one experiment (No. 2). War- 
burg’s observed changes tend to exceed the calculated, while those 
determined in our experiments tend to fall short where they 


THE COLLOIDAL BEHAVIOR OF THE BODY FLUIDS 118 


TABLE I.—CALCULATED Errect oF PH CHANGE ON WATER DISTRIBUTION 
COMPARED WITH Errect OBSERVED BY WARBURG 


Blood constants estimated from hemoglobin content 
(Hb) = 11.3 observed. (B), = 54.2 from equation (16). 





(P), = 0.0279 from equation (17). G, = 1.069 from equation (29). 
(B). = 67.8 from equation (15). (H2O), = 0.745 from equation (30). 
a nn eS ee 
Volume of cells in per 
Ve cent of their volume 
aes at pH 6.8 
—logr Vo 

pHs (from pHe (BP)s (BP) Calculated 

Fig. 1) by equa- |Calculated| Observed 


tion (27) | by equa- y 
tion (27) | Warburg 








6.8 0.04 6.76 4.2 6.5 0.640 100.0 100.0 
7.0 0.07 6.93 4.6 13.4 0.631 98 .6 97.4 
72 0.10 7210 5.0 20.4 0.621 97.0 95.2 
7.4 0.14 7.26 5.3 26.9 0.611 95.5 93.7 
7.6 0.18 7.42 ek 33.4 0.602 94.0 92.2 
7.8 0.22 7.58 Oak 40.0 0.591 92.3 90.5 














baleabsd 

ee 

Sie 
~ 
~ 


RAVER ES 
AG ee aee 


/ 
fi f- 


Pa PS Pe 
ee i 
meee Sele eeeiNe 
pt ON 

T 4. 18 


64 6.6 6.8 


Cell volume in percent of volume at pH 6.4 





16 


: pHs 
Fic. 3.—Cell volumes calculated by Equations 24 and 27 for blood of average 
serum and cell composition observed in experiments. 


deviate from the calculated. The available data appear to agree 
with the predicted values as closely as may properly be expected 
by the limitations of present accuracy in water determinations. 


114 COLLOIDAL BEHAVIOR 


In Fig. 3 the relative cell volume changes resulting from pH 
variations in oxygenated blood, as calculated by equation (27), 
are shown for bloods of varying hemoglobin content. The 
percentage cell volume change caused by a given pH shift is 
greatest when the ratio, cells: serum, is least (hemoglobin lowest), 
because the concentration or dilution of serum, which results from 
the water exchange and tends to diminish the latter, is least when 
the relative amount of cells is smallest. 


ILLUSTRATION OF THE EFFECT OF CO, TENSION CHANGES ON 
THE ELECTROLYTE AND WATER DISTRIBUTION OF 
OxYGENATED BLOooD 


To illustrate the processes involved, we may simplify conditions 
by ignoring minor factors: v7z., the slight amounts of diffusible 
anions other than Cl’ and HCO,’, the osmotic and base-binding 
powers of the serum proteins, and the osmotic effect of the hemo- 
globin. We shall assume the cells to contain only base, hemo- 
globin, Cl, and HCOs, and the serum to be a simple solution of 
bicarbonate and chloride. Equation (10) under these conditions 
becomes simplified to 


[H* |: 1G edn COR [BHb]. 


fHt (Cre CUR: ~ 9(TBCI, + [BHCO,),) 


We shall assume, first, that the COs tension is so low that pH. 
= 7.8, then that it is raised so that pH falls to 6.6. According to 
Van Slyke, Hastings, Heidelberger, and Neill, the alkali bound 
by oxyhemoglobin is indicated by the equation [BHb] = 2.65 
[Hb] (pH — 6.6). Assuming [Hb], = 30 millimols we, therefore, 
calculate at pH, = 7.8 that [BHb] = 95, and at pH, = 6.6 that 
[BHb] = 0. |; 

In Fig. 4 we have indicated the concentrations of the positively 
and negatively charged ions in the cells and serum by the areas 
assigned to each ({Hb’] is indicated in terms of alkali equivalents 
bound). The concentrations of the osmotically active ions are 
indicated by clear areas, while that of the (relatively) osmotically 
inactive [Hb’] is indicated by a shaded area. For simplicity it is 
assumed that the ionization of each electrolyte is complete. It is 
also assumed that at the beginning (Fig. 44) the water content 
of the blood is half in the cells, half in the serum. 











THE COLLOIDAL BEHAVIOR OF THE BODY FLUIDS 115 


The amounts of hemoglobin, base, chloride, and bicarbonate 
indicated are about those found in normal horse blood, except 
that the difference between |B], and [B], in Fig. 4A is somewhat 
exaggerated as a result of ignoring the base bound by the serum 
proteins and the osmotic effect of the hemoglobin. 

The conditions indicated in the four diagrams of Fig. 4 are 
the following: 

1. The conditions represented conform to the three basic laws: 
(a) in both cells and serum the positive and negative ions balance; 





5 
B 
' 
© 
o 
t 
a 
[e} 
£ 
= 
0 
Cells Serum, Cells a Cells — Serurn Cells — Serum 
A 
Bub replacedin Shiftof CI'to , Shift of water 
cells by BHCO; cellsandHCOz —_tocells to restore 
toserumto restore Osmotic equality 
Donnan equilibrium 
ee rr 
pH, 7.8 Results of increasing pco, until pH is lowered to 6.6 


the isoelectric point of oxyhemoglobin 
Fig. 4.—Concentrations of the positively and negatively charged ions in the 
cells and in the serum. 


(b) the ratios cr and TCO" are equal, and conform to the 
simplified form of equation (10) given above; and (c) the osmolar 
concentrations obtained by adding [B+] + [Cl’] + [HCO;’] are 
equal in serum and cells respectively. 

2. Increase of COs tension has lowered the pH, to 6.6, the 
isoelectric point of oxyhemoglobin. The result is that all the base 
formerly bound by hemoglobin as BHb has shifted to BHCOs, 
HCO,’ replacing Hb’. In Fig. 4B, however, only the first of the 
three laws is conformed with. Positive and negative charges 
balance, but the greatly increased concentration of HCO,’ in the 





116 COLLOIDAL BEHAVIOR 


c ] Cc 
cells obviously makes HIGO > are contrary to Donnan’s law. 





The HCO,’ increase in the cells also causes the osmolar concentra- 
tion there to exceed that in the serum. The system is not in 
equilibrium. 

3. To restore electrolyte distribution to conformity with 


Donnan’s law, Cl’ has migrated from serum to cells, and HCO3’ 


in the reverse direction until again HOE = a 





4, To restore also osmotic equilibrium, water has migrated 
from serum to cells until the osmolar concentrations in both are 








Cells Serum 


pH.7.75 pHs 7.08 


Fia. 5.—Relationships observed in defibrinated blood. 


equal. Impermeability of the cell membranes to cations prevents 
diffusion of BCl and BHCO; from cells to serum to assist in the 
restoration of osmolar equality. It must all be accomplished by 
water transfer. The system is now in equilibrium again. 

The processes represented here, for the sake of analysis, as 
though occurring in successive steps must in reality occur 
simultaneously. 

The somewhat more complex changes actually occurring in 
blood, where the alkali-binding power of the serum proteins and 
the osmotic pressure of the hemoglobin enter as appreciable 
though minor factors, are indicated by Fig. 5, which represents 
data obtained from defibrinated blood. X’ is used to indicate 
the undetermined anions. 


THE COLLOIDAL BEHAVIOR OF THE BODY FLUIDS 117 
CALCULATION OF THE ELECTROLYTE AND WATER CHANGES IN 
Bioop DuRING THE RESPIRATORY CYCLE 


We have calculated the changes that, according to our data, 
may be expected to accompany the COz and O» changes of ordi- 
nary respiration. 


Total 
it BHCO, Per cent ey Percent 


mM. > of blood E of blood HCO 
Bed perkg. HOin. -s Cland HCOz mM. per tensa 
blood sas serum E inserum kg.blood ‘mm. 

ee ee ple oe a Sa 
28° 2: 580 ' c 1.50 
3 E83 
E f os 5 100 
24 es | 
Bt abla ree is 
vbn= (40 150 
3422 he 
5 . 2 rorid! 5 
59.0 chee ps MY 
2 i541 9 49 
: oe peel 
2 595 01 ae ey, [20 
: > 120 
1.30 
18 7.30 
19 600 
17 > 140 oe 
18 
1.20 


Fig. 6.—D’Ocagne-Henderson nomogram showing calculated relationships for 
arterial and venous blood. (From Van Slyke, Wu, and McLean). 


In Fig. 6 we have indicated the relationships on a D’Ocagne 
nomogram, of a type that was devised by L. J. Henderson. A 
straight line, drawn across the chart, and cutting the lines repre- 
senting oxygen and CO, tensions at any given point, cuts the 
lines representing (BHCO3;),, (HbO:2), pH;, pH., etc., at points 
indicating the values these respective quantities have under the 
given Pco, and po,. Such a line can be drawn because all the 
other variables in a given blood are dependent on these two. 
Over the range used, the chart is quite exact. Details of the 


118 COLLOIDAL BEHAVIOR 


calculations involved in the construction of the nomogram may 
be obtained from the original paper of Van Slyke, Wu, and 
McLean. Comparison with available data, particularly of 
Doisy and Beckmann, for arterial and venous blood, indicates 
as close agreement with Fig. 4 as could be expected. 


ELECTROLYTE DISTRIBUTION BETWEEN BLOOD SERUM AND 
TRANSUDATES AS A FUNCTION OF THE ALKALI BOUND 
BY THE PROTEINS 


Loeb, Atchley and Palmer have performed experiments indi- 
cating that the membranes separating the blood serum from the 
fluids in the body cavities and intercellular spaces have the same 
permeabilities as collodion for the substances present. 

Under these conditions the Donnan distribution would require 
expression by an equation including Na and K among the diffus- 
ible ions, instead of excluding them, as does equation (3). Expres- 
sing the distribution ratio of monovalent ions between serum and 


fluid as r,s, the relationship would, theoretically, be 
. [AL BE 


HE TAT) o [Bet a ata 


AS] eee : : ae 
when rer indicates the ratio of the osmotic activity of any mono- 
f 





(31) 


/ 


valent anion, or sum of anions, in the serum to the osmotic 


Aes ee ; Be hae Si 
activity of the same ion or ions in the fluid, while 5 A hasasimilar 


significance for the cations. If in place of [B*], and [B*]y we 
substitute their values from equations (1) and (2) we obtain 
IA, + (BPI, 
1 TAT 








(32) 


If we substitute ee for A;, and solve for rs, we obtain 


sf 

pipet Bl gets VBP}; + 4[A]. ((A]. + [BPI.) (33) 

7 2([A]. +[BP].) 
We have recalculated in Table II Loeb, Atchley, and Palmer’s 
solute , solute 
volume ~ water 
by estimating the grams of water per liter to be 990 — 0.8 P, 
where P represents grams of protein per liter. (This may be 





ratios 








data, transposing the concentrations from 


THE COLLOIDAL BEHAVIOR OF THE BODY FLUIDS 119 


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120 COLLOIDAL BEHAVIOR 


taken as a fairly close approximation, unless abnormal amounts of 
fat or other solids are present.) The [BP], and [BP]; values are 
calculated on the assumption that the proteins of human serum at 
pH 7.4 bind the same amount of alkali per gram as the proteins of 
horse serum (the slight difference in pH between plasma and 
fluid may be neglected). At this pH the formula (equation (54)) 
[BP] = 0.068 [P] (pH — 4.80) becomes [BP] = 0.177 [P]. The 
arterial HCO; values are estimated by subtracting 2 millimols 
per liter from the values found in the venous serum. 

The estimated [Cl], : [Cl]; ratios found coincide with the cal- 
culated r,, values nearly within the limit of experimental error. 
The [HCOs],: [HCOs]; ratios are all higher than the calculated r, ; 
when the venous values for [HCOs], are used; but the estimated 
arterial values for [HCOs], yield [HCOs], : [HCOs3]; ratios which 
agree with the calculated r,y as closely as could be expected, when 
the possible magnitude of the error involved in assuming a con- 
stant difference between arterial and venous CO, is considered. 

The [Na]; : [Na], ratios agree, in six out of seven cases, with the 
calculated r,; values within the rather wide limit of error assigned 
by the authors to the Na determination. The [K];:[K], ratios 
are altogether lower than the calculated r;;, and are very irregular. 
The source of the deviation and irregularity of the K ratios is at 
present uncertain. Considering the minute amounts of K pres- 
ent, it appears possible that the irrregularities may lie in the 
micro method used for the determination. 

The irregularity of the potassium ratios, and the necessity for 
using estimated water and arterial HCO; values, make it impos- 
sible to consider the presence of a Donnan equilibrium between 
blood serum and edema fluid as quantitatively demonstrated 
with satisfactory accuracy. It appears probable nevertheless . 
that the degree of agreement found between the calculated r,; 
values and the ratios for Cl, HCO; (arterial), and Na ismore than 
fortuitous; that it affords support for Loeb, Atchley, and Palmer’s 
conclusion that “the relationships between serum and edema fluid 
result from a simple membrane equilibrium, influenced in part by 
the proteins present.” 

If the membranes separating blood serum from other extra- 
cellular fluids are permeable to all electrolytes present in amounts 
of quantitative importance except protein, it follows from the 
Donnan theory that the serum, containing more protein than the 


THE COLLOIDAL BEHAVIOR OF THE BODY FLUIDS 121 


other fluids, must, when at equilibrium with them, show a positive 
osmotic pressure. While the basic equations of the form of 
equations (1) and (2), and of equation (8) modified to include Na, 
hold for such a system, equation (4) and its derivatives expressing 
osmolar equality do not, so long as the serum volume is limited. 
The preponderance of the osmolar concentration even of the 
diffusible ions, on the side containing non-diffusing ions, when the 
latter are entirely on one side of the membrane and infinite 
volume change is excluded, has been theoretically shown by 
Procter and Wilson. 

If the non-diffusible electrolyte (protein) also has a measurable 
attraction for water, the osmotic preponderance on its side of the 
membrane is still further increased. If serum and a transudate 
relatively poor in protein are separated by membranes permeable 
to all the non-protein ions present in quantitatively important 
concentrations, viz., Nat, Cl’, and HCO’, but impermeable to the 
protein, we may, therefore, expect the serum to exhibit a higher 
osmotic pressure than the edema fluid. With the osmotic 
pressure tending to draw water into the serum, it appears that 
other forces are responsible for the passage of fluid in the direction 
from the blood to the serous cavities and intercellular spaces. 


CONCLUSIONS 


Without assumption of other laws than those known to hold for 
physico-chemical relationships in dilute solutions, it has been 
possible to explain the distribution of water, chloride, and 
bicarbonate between the blood plasma and cells, and to predict 
the direction and magnitude of the shifts caused by reaction 
changes. The latter are completely accounted for by the changes 
in base bound by the proteins with changing pH. Assumptions 
of “hydration” or ‘‘adsorption”’ phenomena have not been 
required. ‘There is evidence that the same laws govern the 
distribution of electrolytes between the blood and other body 
fluids, although forces other than osmosis (e.g., pressure in the 
vascular system) play a part. 

In conclusion, it is of interest to point out that, while we have 
information concerning the permeability of some of the body 
membranes to certain substances, and can thereby demonstrate 


122 COLLOIDAL BEHAVIOR 


conformity of the observed diffusion phenomena with Donnan’s 
law and the classical laws of osmosis, yet we are in entire ignor- 
ance as to the cause of some of the observed differences in per- 
meability. We do not know why the membranes of the 
erythrocytes are permeable for water and anions, but not 
for cations. 


REFERENCES 


Dorsy, E. A. and Eaton, E. P.: J. Biol. Chem., 47 (1921), 377. 

Donnan, F. G.: Elektrochem. Z., 17 (1911), 572. 

GirBer, A.: Jahresb. Thierchem., 25 (1895), 165. 

Hampurcer, H. J.: Lancet, 2 (1921), 1039. 

Hiner, G. and Gansssr, E.: Arch. Physiol. (1907), 209. 

Kramer, B. and Tispatt, F. F.: J. Biol. Chem., 58 (1922), 241. 

Lors, J.: “Proteins and the Theory of Colloidal Behavior,’ New York, 
1922. 

Lorn, R. F., Atcuury, D. W. and Patmsr, W. W.: J. Gen. Physiol., 4 
(1921-22), 591. 

Micuaruis, L., cited in Héser, R.: “Physikalische Chemie der Zelle und 
der Gewebe,” Leipzig and Berlin, 1914, p. 330. 

NrvHAvsEn, B. 8.: J. Biol. Chem., 61 (1922), 435. 

Nevuavussn, B. S. and Marsuaut, E. K., Jr.: J. Biol. Chem., 53 (1922), 
365. 

Procter, H. R. and Witson, J. A.: J. Chem. Soc., 109 (1916), 307. 

Srartine, E. H.: J. Physiol., 19 (1895-96), 153. 

Van Styxz, D. D., Hastinas, A. B., HemeEnsercer, M. and Nei, J. M.: 
J. Biol. Chem., 54 (1922), 481. 

Van Suyks, D. D., Wu, H. and McLzay, F. C.: J. Biol. Chem., 56 (1923), 
765. 

Warsura, E. J.: Biochem. J., 16 (1922), 153. 

Zucker, T. F. and Gurman, M. B.: Proc. Soc. Exptl. Biol. Med., 19 (1921— 
22), 169. 


CHAPTER V 


THE KINETICS OF DISPERSE SYSTEMS 
By 


EK. FRANKLIN BuRTON 


Probably the most interesting contribution of the study of 
colloidal solutions to the domain of pure science has been the 
visual confirmation which these solutions supply to the hypothe- 
ses of the kinetic theory of matter. This latter theory, in so far 
as it deals with gases, has been in a fairly complete state for some 
time, but the difficulties inherent in the application of the kinetic 
theory to liquids and solids produced many doubters. In the 
case of the disperse systems we have, fortunately, a distribution 
of particles which can be rendered visible and which have supplied 
incontrovertible evidence of the reality of that molecular motion 
in liquids demanded by the kinetic theory. 

From the point of view dealt with in this chapter, we shall 
consider the disperse systems, which are suspensions in liquid 
media of liquid globules or solid particles, the linear dimensions 
of which lie between the limits 10—° and 10-7 em. (or 0.1uand luy). 
The physical conditions which fix these two limits are: (1) for 
ordinary solid particles, 10-5 cm. is the diameter of the largest 
spherical particle which will remain in suspension for an unlimited 
time, and (2) 10-7 cm. is about the diameter of the smallest (sup- 
posedly spherical) particle which has been made visible by means 
of the most highly developed ultramicroscope. We have, then, in 
these systems evenly distributed clouds of small particles in a 
liquid atmosphere—particles small enough to be relatively com- 
parable to molecular sizes and, consequently, small enough to 
partake to a noticeable extent of molecular motions. 

The kinetics of these systems are important from the point of 
view of the following kinds of motion, which will be dealt with 
here: (1) the settling due to gravitation or artificially produced 
by centrifuging, (2) the Brownian movement—the continuous 

123 


124 COLLOIDAL BEHAVIOR 


zigzag motion of fine particles in suspension, (3) the mobility 
of such particles in an electrical field, due to the possession of 
electrical charges, and (4) a possible motion influencing their 
distribution, due to the mutual action of the particles. 


SETTLING DUE TO GRAVITATION OR CENTRIFUGING 


When a sphere (radius, a cm.) is moving through a liquid 
medium with a velocity v cm. per second, the frictional force 
between particle and liquid is, according to Stokes’ law,! given by 

F = 6rnav dynes (1) 
where 7 is the coefficient of viscosity of the liquid in the c.g.s. 
system. In the case of such a spherical particle falling under 
gravity through the liquid medium, a limiting velocity is attained 
when the above frictional force is just equal to the gravitational 


: ee. 
force acting on the particle, viz., 3 mae (d, — de)g, where d; and dz 


are the densities of the material of the particle and the medium 
respectively. Thus, we have, for the limiting velocity v., the 
relation 


6rnav, = om a’(d, — de)g 


or 
2 a?(dy a d2)g 

The following table shows the values of v, for various sizes of 
spherical silver particles, falling freely through water at ordinary 
room temperature, assuming the above law to hold. 





Usne= 


TABLE | 

















Radius, Time to fall 1 cm. 
‘ Vel. cm. per second : 
centimeter seconds 

| 

0.010 20. 0.0500 

0.0010 2.00 5.00 

0.00010 0.0020 ; 500. 

0.000010 0.000020 50,000. 1g day) 

0.000001 0..0000002 5,000,000. (58 days) 
| 


eee 
1 Cf. Lams: “Hydrodynamics,” 3rd ed., 1906, pp. 551-554. 


THE KINETICS OF DISPERSE SYSTEMS 125 


From this table it is apparent that the rate of settling for 
particles included within the above limits (0.1u to luu) will be so 
small that it would be easily masked or neutralized by convection 
currents and molecular shocks. As the particles considered are 
taken smaller and smaller, the gravitational fall becomes less and 
less important in comparison with the motion caused by molecu- 
lar bombardment. Measurements of such settling rates as 
indicated in the table may be used to determine the values of 
a for the particles. 

By means of the centrifuge the force tending to pull the 
particles to the bottom of the containing vessel can be made to 
attain a value several times that due to gravitation. When a 
particle is revolving in a circle, as in the case of one of these 
colloidal particles in a sample which is being centrifuged, the 
force which must be exerted upon it in order to keep it in the same 
circle (say of radius r cm.) is given by 

F, = mw*r 
where w is the angular velocity of the particle about the circle. 
Consequently, F; is a measure of the force tending to throw the 
particle away from the center—the so-called centrifugal force. 
That is, we have produced in the suspension an artificial gravita- 
tional force, tending to throw the particle to the bottom of the 
vessel, given by 
F, — 2 ra>(dy “7 do)w?r (3) 
which will produce a corresponding limiting velocity ve given by 
Stokes’ law as follows: 
Vo = - ma wr (4) 
The ratio of this limiting value to that caused by gravitational 


settling is for the same sample: 
ee 
snag (5) 


For example, for a particle 10 cm. from the center of rotation 
and an r.p.m. of 1,000 
eo ea xX LOC. ps. Units 
and 
Vo = 11380. 


126 COLLOIDAL BEHAVIOR 


From this it is apparent that the centrifuge under conditions 
of steady motion can be used to enhance the gravitational 
settling and enables one to determine the value of a from much 
smaller particles than in the case of gravitational settling alone.’ 


THE BROWNIAN MOVEMENT 


An English botanist, Robert Brown’ (1830), first observed 
that a curious zigzag motion was shown continually by small 
inanimate particles suspended in water. His first observations 
were made on pollen dust, a very fine powder which remained in 
suspension in water for a great length of time; similar intermin- 
able motions were afterward recognized in such suspensions as 
clay, powdered glass, etc. This kind of zigzag motion had been 
observed many years before Brown’s time in suspensions of 
animalcules but was tacitly assumed to be a consequence of life 
in these small beings. 

Many explanations of this motion were suggested—convection 
currents, external vibratory motions, light, electrical charges, 
surface tension, etc.—but experiment has shown that this motion 
is unmistakably due to resultant momentary shocks due to bom- 
bardment of the particles by the molecules of the liquid medium 
in the course of their ordinary thermal agitation. An applica- 
tion of the fundamental principles of the kinetic theory of 
matter enables one to calculate the displacement which a particle 
of given radius will undergo in any given time, in terms of the 
ordinary constants of the medium. 

Such calculations have been carried out independently by 
Einstein, Smoluchowski,® and Langevin.® An outline of the for- 
mula due to Einstein will be given here; the other methods have 
been described elsewhere. In all of these methods two funda- 
mental assumptions are made: (1) in the motion of these particles, 

2 SvepBERG: Colloid Symposium Monograph, University of Wisconsin, 
1923, p. 75. 


3 Burton: ‘Physical Properties of Colloidal Solutions,” 2nd ed., p. 51 
et seq. 

4 EINSTEIN: Ostwald’s Klasstker exakt. Wissen., 199; Ann. Phys., 4 (1905), 
549; 4 (1906), 371; Z. Elektrochem., 14 (1908), 235. 

5 SMoLucHowsEI: Bull. Intern. Acad. Sci. Cracovie, T (1906), 577; Ann. 
Phys., 4 (1906), 759. 

6 LANGEVIN: Compt. Rend., 146 (1908), 530. 


THE KINETICS OF DISPERSE SYSTEMS 127 


the kinetic energy of the colloidal particle is equal to the mean 
kinetic energy of the molecules of the medium—the law of the 
equipartition of energy—and (2) in the resultant motion of the 
particles the frictional force opposing the motion is given by 
Stokes’ law, F = 6ryav. The first assumption allows one to 
introduce the ordinary kinetic constants represented in the 
kinetic energy of a molecule, while the second one enables one to 
connect up the velocity of the particle with its radius. 

Kinstein’s method depends upon calculating the diffusion 
constant D of the colloidal particles by two independent methods, 
and equating the two expressions. The diffusion constant 
is defined as follows: If we choose as the axis of « the 


direction of the diffusion, and if is the rate of change along x 


of C, the concentration in mols per cubic centimeter of the 
diffusing substance, then the quantity of dissolved substance 
transported per second through 1 sq. cm. perpendicular to the x 


ae d . 
axis is given by D. - mols. That is, the coefficient D is equal 


to such a number of mols when the value of a Be] 


Calculation of D from Osmotic Considerations—Let a cylin- 
drical vessel S (with axis parallel to axis of X) be divided into 
two parts, A and B, separated by a semi-permeable wall G (Fig.1). 





If C4>Cz, force must be exerted on the wall G towards the 
left in order to maintain equilibrium, the size of the force per 
square centimeter being given by the difference between the 
osmotic pressures. If this force is not exerted, G will move 
toward the B compartment until the concentrations C, and Cz 


128 COLLOIDAL BEHAVIOR 


are equal. If the wall G is not inserted, osmotic forces will 
regulate the equalization of concentrations. 

In Fig. 2 let the cross-section of S be 1 sq. em. Consider the 
osmotic forces acting on the dissolved substance between two 
neighboring planes # and EL’ a distance dz apart. Let p and p’ 
be the osmotic pressures at # and E’ respectively. p — p’ = 
osmotic force which acts on volume (dz X 1) cc., that is, this is 
the force which acts on the substance dissolved in this volume. 


Thus oe = osmotic force on material dissolved in 1 cc. = say, K. 





Fra, 2: 


If the osmotic pressure satisfies the equation 
pn td Oh (6) 


where C = number of mols of dissolved substance per cubic 
centimeter of solvent, then 


ii = — + = —RP. (7) 


If N = number of molecules of a substance in 1 mol, there 
will be NC molecules of solute per cubic centimeter of solution, 
and, since K is the osmotic force on all the dissolved material in 


1 cc., we have the force per molecule equal to ae We may treat 


each particle in a colloidal solution as a large molecule, and, on 
the kinetic view of osmotic pressure, consider the osmotic force 
per particle as equal on the average to the osmotic force per 


molecule. Therefore, = = the force due to osmosis tending 


to urge a given particle toward the region of increased concentra- 
tion. 


Ee ™ 


THE KINETICS OF DISPERSE SYSTEMS 129 


If the diffusing particles are spheres, large compared with 
molecules, we may use Stokes’ expression for the frictional force, 
opposing the motion of such sphere, v7z., 


F = 6ryav 
F haste 
ee 6rna 6rna NC 
1 I dC 
Mei UNG cand: (8) 


But from the definition of the diffusion coefficient D, we have 


om = the quantity in mols of the dissolved substance which 


passes through a square centimeter area perpendicular to the X 
axis per second. This equals vC. 








Therefore, 
dC 1 1 dC 
Da, lima NORTE 
and 
eee 
~ 6rna N- (9) 


Calculation of D from Simple Molecular Motions.—The 
molecules of the medium and of the dissolved substance have a 





HIGs 3. 


non-uniform irregular motion. Let the motion be along the axis 
of the same cylinder of solution, the axis of the cylinder being 
the axis of X (Fig. 3). 

Consider particles at the plane EH at time?t. Ina time interval, 
7 sec. taken so small that the distribution of concentrations does 
not change appreciably, these particles will have traversed dis- 
tances A;, Ae, A3, etc., which are positive and negative at random. 


130 COLLOIDAL BEHAVIOR 


We assume that this motion is affected by the motion of the 
molecules of the medium but that, on account of the dilution, 
there is little mutual action among the particles themselves. 

The determination of the value of the diffusion coefficient D 
depends upon calculating how much of the material diffuses 
through a plane area of unit cross-section in the time 7, if the 
magnitude of the motion A parallel to the axis of the cylinder is 
known. Suppose the particles move an average distance A, 
one-half in a positive direction and one-half negatively. Through 
the plane E, there will pass only those molecules which at the 
beginning of the time 7 are distant from EH by an amount less than 
A, 2.e., included between the plane H’ and E”. 

Taking those between H and EH’, since one-half move to the 
right and one-half to the left, the number passing through # 
from this side will be 

1eCiA (10) 
where C, is the mean concentration in region FE’, 1.e., the con- 
centration at the plane M midway between # and LH’. 

Similarly, for the region HE’’, the number passing through # 
in time 7 will be 
where C2 is the concentration along M>. 

The net diffusion through F per square centimeter in time 7 is 
1g A(C; — C2) in the positive direction of X. 

If the X coordinate of E is 2, 


Ce — C1 dC 
A dz 
Therefore, 
E24 
da 
and 
se ane dC \ 
AC, << C2) = —/4 oe (12) 


.. The diffusion per second through unit area of H# is 


A? aC dC 
Lg i ee 
(2 > dt eT 
Therefore, 
A2 
D =") (13) 


THE KINETICS OF DISPERSE SYSTEMS 131 


Combining this last equation and equation (9) we have 


raed Me 
«| te glen Bane aly ape 
6rya N ce oe, 
which gives the general formula for the Brownian movement: 
Rilke ab 
oA i ae tn a 
A Aeron T ba) 


where A is the average distance a spherical particle of radius a 
will move in time 7 sec. through a liquid of viscosity 7. 

R = 8.31 X 10’ c.g.s. units 

N= 6% 1073 

T = absolute temperature 

Space will not permit recording an account of the experi- 

mental tests of this formula carried out by various workers. 
The agreement between observed and calculated values is close 
enough to leave no doubt as to the truth of the kinetic explana- 
tion of the Brownian movement. 


Tue THEORY OF FLUCTUATIONS 


The kinetic explanation of the Brownian movement has received 
additional confirmation from the theory of fluctuations, as worked 
out by Smoluchowski.’ In the ordinary treatment, when one 
speaks of uniformity of density and temperature in a liquid 
or gas, one does not consider the motion of individual molecules as 
being constant from molecule to molecule; one takes the average 
over a volume containing an immense number of molecules. 
Since molecular agitation intervenes in all physical phenomena, 
fluctuations should accompany all apparent equilibria. 

Smoluchowski deduces from this theory the fluctuation in its 
position of a colloidal particle and obtains a formula which is in ~ 
agreement with experiment. 


PERRIN’S DISTRIBUTION LAW 


Perrin® has deduced theoretically an expression for the law 
of distribution of colloidal particles, so as to show how the 
concentration of particles varies with the depth of the point 
below the surface. Equating the resultant osmotic effects due 


7 SmoLtucHowskI: Acad. des. Sc. de Cracovie, Dec., 1907. 
8 PprrRin: ‘‘Atoms’’ (tr. by Hammick), 2nd ed., 1923, p. 89, et seq. 


132 COLLOIDAL BEHAVIOR 


to the particles in suspension which tend to produce uniformity 
of distribution to the resultant gravitational force, he obtains 


the formula 
jaa ote 
ee wane 


where n and n, are number of particles per cubic centimeter at 
depths h and h, below the surface, V and d are the volume and 
density respectively of a particle, and w is the density of liquid 
medium. This equation shows that the concentration of the 
particles of a colloidal solution should increase in an exponential 
fashion as a function of the depth, or, in other words, as the depth 
increases in arithmetical progression, the concentration of the 
particles should increase in geometrical progression. Perrin 
tested this law for depths up to 0.1 mm. and found it to hold. 
It is doubtful if the law holds for a much greater depth (see 
Burton® and Porter?®). 

From his measurements of ener of n, h, V, and d, Perrin 
deduced values of NV, the number of molequiees in a gram mnleente 
of a substance, which agree well with other determinations of N 


‘Vid — w)g(h — h.) (16) 


MoTION IN AN ELECTRICAL FIELD 


If two electrodes maintained at a difference of potential are 
inserted in a vessel containing a colloidal solution, as a general 
thing the particles of the solution will move to the positive or 
negative pole; consequently, we say that these particles are 
negatively, or positively, charged. This phenomenon is not 
peculiar to colloidal particles in suspension, as many years ago 
experiments were carried out on the mobility, in an electric 
_ field, of particles of starch, platinum black, finely divided metals, 
graphite, quartz, etc., suspended in water and other liquids. 
As a result of such experiments it was found in every case when 
water was the suspending medium that the particles moved to the 
positive pole; this led to the statement that ‘‘in water all bodies 
appear through contact to become negatively charged, while, 
through rubbing against different bodies, the water becomes 
positively charged.” 


®° Burton: Proc. Roy. Soc (London), A, 100, 705. 
7° PorTER and Hxepeers: Trans. Faraday Soc., 18 (1922), 1 


THE KINETICS OF DISPERSE SYSTEMS 133 


This conclusion is in keeping with the observations of the 
motion of water through capillary tubes or porous cups, when an 
electrical field is maintained through the capillary openings. 
In this case, the water moves toward the negative electrode, 
from which we conclude that the wall of the capillary becomes 
negatively charged while the water itself becomes positively 
charged, the so-called electroendosmose effect (see Perrin,!! 
Elissafof,!* Briggs-Bennett-Pierson'*). In the case of the parti- 
cles in suspension the wall moves through the liquid, while in 
using capillary tubes the wall is fixed while the liquid moves. 

Extended work on the mobility of colloidal particles has 
not supported the statement quoted above; some colloidal 
particles move towards the positive electrode and others towards 
the negative electrode. ‘Taking into consideration the recent 
results of many workers, we may divide colloidal solutions and 
suspensions into two classes, anionic and cationic, according as 
the particles in solution move to the anode, 7.e., are negatively 
charged, or to the cathode, 7.e., are positively charged. 


SOLUTIONS IN WATER 


Anionic Cationic 
1. Sulfides of arsenic, antimony,and 1. Hydrates of iron, chromium, 
cadmium. aluminum, copper, zirconium, 
2. Solutions of platinum, silver, cerium, and thorium. 
gold, and mercury. 2. Bredig solutions of bismuth, lead, 
3. Vanadium pentoxide. iron, copper. 
4. Stannic acid and silicic acid. 3. Hoffman violet, Magdala red, 
5, Aniline blue, indigo, molybdena methyl violet, rosaniline hydro- 
blue, soluble Prussian blue, chloride, Bismarck brown, 
eosin, fuchsin. methylene blue. 
6. Iodine, sulfur, selenium, shellac, 4. Albumen, hemoglobin agar. 
resin. 5. Titanic acid. 


7. Starch, mastic, caramel, lecithin, 
chloroform, agar-agar. 

8. Silver halides. 

9. Various oil emulsions. 


The grouping of substances in the above list suggests that 
in the majority of cases the so-called “charging by contact 
between the solid and water”’ is in reality intimately connected 


11 PpRRIN: J. chim. phys., 2 (1904), 607; 3 (1905), 50. 
12 ELIssaFOF: Z. physik. Chem., 79 (1912), 385. 
18 Briges, BENNETT and Pierson: J. Phys. Chem., 22 (1918), 256. 


134 COLLOIDAL BEHAVIOR 


with the chemical constitution of the substances involved (see 
Kruyt?*). 

Various methods!® have been used to measure experimentally 
the mobility of these particles in an electric field (in centimeters 
per second per volt per centimeter). The most satisfactory 
method is by means of a U-tube, such as used by Nernst, Whetham, 
Hardy, and others. Table II gives the values of the mobilities 
for various disperse systems together with calculated values of 
the difference of potential between the disperse phase and 
the medium. 

It is a remarkable fact that the mobility of these colloidal 
particles is just about the same as that of electrolytic ions, with 
the exception of the fast-moving hydrogen and hydroxyl ions, 
although the sizes of the colloidal particles are many times the 
size which one usually associates with the ions. In this con- 
nection it is interesting to note that Lamb’s theoretical formula 
for the mobility of a particle leads to the following equation: 

Ar nv 
where 
V = difference of potential between particle and liquid 
K = dielectric constant of liquid 
n 
xX 
v 


viscosity of liquid 
= applied electric field 
limiting velocity of particle in the given field 


II 


From this we see that for a given V and for a given liquid medium, 
under constant conditions, the mobility of the particle is inde- 
pendent of the radius of the particle. The above formula is 
deduced by equating the electrical force acting on the particle 
to the fractional resistance as given by Stokes’ law, and, con- 
sequently, will be true only for particles for which Stokes’ law is 
applicable, 7.e., for particles the diameters of which are large 
compared to the mean free path of the liquid molecule. This 
suggests that the ordinary electrolytic ion is an entity to which 
Stokes’ law may be applied. 


14 Kruyt: Nature, June 16 (1923), 827. 
* Burton: “Physical Properties of Colloidal Solutions,’’ 2nd ed., p. 132, 
et seq. 


THE KINETICS OF DISPERSE SYSTEMS 


135 


TaBLeE IJ].—MOoBILitTIES OF THE DISPERSE PHASE 1N VARIOUS AQUEOUS 


SOLUTIONS 





Disperse phase 


Mobility in centi-| pP.p. in volts 





Suspensions: 
PE VCODOCMIITIE tiara ay 5 ae 8 eco 


Suspensoids: 

Arsenious sulfide............... 
PEeTUISSI eT ED EUs tas sic us ede ast es 0s 
iprseram DIUCr es d.e ae. fae 3 

Gold (chem Sprep.).......5..%... 
Goldwlehem* Dreps) hi... ce. aes 
Gold (chem. prep. and Bredig). . 
GoladbGhredigicrre sii ne dele. yates 
ieabradbavthaies, ca © tua.s Sieh aheae See cee 
Pistinum (Bredig)..--s........ 
Platinum (Brédig) <7.) wc. as... 
Platimunm CBredig asus 2 we co § 
DibVersV Brealey es css. pele. = s+ 
DilversCDredia) sian eee. ss ocetenss 
Wiereurys (bredig).-..c 264 45«- << 
Silver redie) vscce «cose dick oe 
Bismuth CSredie | we ees se faeces ols 
MSCACECETOCOUG ee cee eo ickclpelaic p50» « 
Tron (CBredigyaw. ahead oes sees ou 
Merricsny-Groxide: wo. eee pele hs 
Ferrie hydroxides sy. -¢)s.)a « «0s 
EPASG LODITac ace 6 ace es ee 


Hp SOs CrlOOulin ss. seats tee se 
a POm Globubitnn sete aint ols = 
Emulsions: 
ELVOrOCATDOUGOUS few ye Stace ss 
Spec. acid-free oil.............. 
ASIC -—PrECLOLL Ace es 2 aesunici es ae ese dis 
PRGUTATSELD, co cs se eo 
Gylinderi oie... ene ess 
Water-soluble oil............... 
Aniline, fresh distilled.......... 
Cilorolonliy oe ses oe la sk 
Cammisut tay etek ets ss 
Mastixharz........ 
Electrolytic ions: 
Organic compounds (high mo- 
Lecwlare weit) pero sis .se w i orn 
PPV GTOTOUN Tale ome auisee fees FG io 
ET yOTOXIGENC—) yeni. guns see se 
Momlorine.(—).7.- 0.5 se este ese 























meter per second | between the Authority 
per volt per centi- | disperse phase 
meter X 1075 and medium 
— 25.0 —0.035 Quincke 
— 30.0 —0.042 Whitney and Blake 
— 40.0 —0.056 McTaggart 
— 22.0 —0.031 Linder and Picton 
—40.0 —0.056 Whitney and Blake 
—41.5 —0.058 Burton 
—40.0 —0.056 Whitney and Blake 
Coy Gs lbs tay RoW ae all Pee heat! Galecki 
— 26.0 — 0.036 Rolla 
—21.6 —0.030 Burton 
— 30.0 —0.042 Whitney and Blake 
— 24.0 —0.0384 Rolla 
—20.3 —0.028 Burton 
—20.0 to —40.0) _............ Svedberg 
—=32)09tO —38.0) ~~ .s5. ess Cotton and Mouton 
— 20.0 —0.028 Svedberg 
— 25.0 —0.035 Burton 
— 23.6 —0.033 Burton 
11.0 0.015 Burton 
125.0 0.017 Burton 
19.0 0.027 Burton 
30.0 0.042 Whitney and Blake 
5200 0.073 Burton 
—19.8 to —22.9 —0.031 Hardy 
— 9.0to —11.5 —0.015 Hardy 
Fiat 0.100 Hardy 
—18.5 —0.026 Hardy 
— 23.0 — 0.032 Hardy 
— 43.0 —0.060 Lewis 
— 37.2 —0.052 Ellis 
—32.4 —0.045 Ellis 
—29.3 —0.041 Ellis 
— 27.0 —0.038 Ellis 
—48.0 —0.067 Ellis 
—31.1 —0.043 Ellis 
—10.0 —0.014 Ellis 
—18.1 —0.025 Ellis 
—17.7 —0.024 Ellis 
20.0 
329.0 
180.0 
68.0 





136 COLLOIDAL BEHAVIOR 


Experiments carried out by Currie!’ to test the influence of the 
viscosity of the medium on the mobility of electrolytic ions and 
colloidal particles show that the value of v is always such that nv 
is constant for given solutions with fixed value of X. 

The existence of this mobility of particles in an electric field 
raises in our minds the question of the mechanism by which these 
particles become charged. On the one hand, we have the purely 
mechanical suggestion of ‘‘charging by contact’’—words used as 
a cloak for ignorance. At the other extreme we have Loeb?’ 
maintaining that the whole action is explicable as a purely 
classical chemical reaction. Whatever the language used, 
we must recognize that the possession of a negative charge by a 
surface of a wall (or of a particle) means the existence of a 
supernormal collection of electrons on that surface, while the 
possession of a positive charge means a corresponding deficiency 
in electrons. This derangement of the normal distribution of 
electrons may, in the case of contact of two solids, e.g., fur and 
ebonite, be due to the direct transfer of electrons from one body 
to the other; however, in the case of contact between solid and 
liquid, since the transfer of charges takes place by means of the 
motion of ions, the most rational explanation seems to be that 
the solid surface becomes charged by means of an absorption of 
ions of one sign—a result which may be brought about by the 
interposition of a chemical reaction at the surface or by a mere 
selective adsorption of one kind of ion by the solid surface. Loeb 
seems to push his results too far when he denies the existence of 
surface adsorption in solid colloidal particles merely because the 
water-permeable protein particles with which he experiments 
indicate quite conclusively the existence in them of a reaction 
throughout their whole volume (see McBain!'® on soaps). ; 


COAGULATION OF SOLS BY EKLECTROLYTES!? 


The separation of colloidal solutions into suspensoids and 
emulsoids is markedly justified by wide differences in sensitive- 
ness to added electrolytes. As a general rule, suspensoids are 

16 To be published. 

17 Lors: “Proteins and the Theory of Colloidal Behavior,” 1922. 

18 McBain: Third Coll. Chem. Report Brit. Assoc., 1920, p. 2. 

19 BurToN: ‘‘Physical Properties of Colloidal Solutions,” 2nd ed., p. 155, 

et seq. 


THE KINETICS OF DISPERSE SYSTEMS 137 


precipitated by extremely small additions of electrolytes, while 
the emulsoids are affected by comparatively strong solutions only. 

Coagulative Powers of Electrolytes—To a given volume 
of colloidal solution is added a quantity of electrolyte sufficient to 
produce coagulation (precipitation) of the disperse phase; if the 
molecular concentration of the electrolyte in the mixture be c, 


Me 
then 28 called the coagulating power of the given electrolyte on 


the given sample of the colloid. Among several samples of the 
same colloid one should express the coagulating powers of dif- 
ferent electrolytes in terms of the necessary concentration per 
gram of the disperse phase per cubic centimeter of sol. 

Two remarkable results are evident on comparing the coagula- 
tive powers of various electrolytes on colloids of different kinds; 
first, the coagulation depends almost entirely on the ion bearing a 
charge of sign opposite to that of the colloidal particle, and second, 
with solutions of salts trivalent ions have, in general, immensely 
greater coagulative power than divalent ions, and the latter, in 
turn, much greater than univalent. Acids and alkalies in particu- 
lar cases act more strongly than the corresponding salts. 

Systematic work on this phenomenon was first undertaken by 
Schulze, and Linder and Picton, from a chemical point of view. 
The coagulative powers of different salt solutions were deter- 
mined by the former for arsenious sulfide and antimony sulfide, 
and by Linder and Picton for arsenious sulfide; their conclusion 
was that this coagulative power depended solely on the valency 
of the metal ion, 7.e., the ion bearing a charge opposite to that 
on the sulfide particle. 

On examination of the results of experiments of coagulative 
powers, one is struck by the remarkable differences which, as a 
general rule, are apparent in the coagulating powers of univalent, 
divalent, and trivalent ions. The earlier workers apparently 
looked upon the differences existing between two different ions 
of the same valency as experimental errors and were led to suggest 
the two laws indicated above: (1) that the coagulating power of an 
electrolyte depended only on the ion bearing a charge opposite in 
sign to that on the colloidal particle, and (2) that the powers of 
univalent, divalent, and trivalent ions were in the ratios which 
may be expressed, as suggested by Whetham, by the ratios 


138 COLLOIDAL BEHAVIOR 


1: x: x, where z is a constant. As much of the early work on 
coagulation seems to support this result, it has come to be 
called the Schulze-Linder-Picton law of coagulation. Taking 
the averages of recorded results, we have the following numbers 
for the ratios: 


Linder and’ Picton .2.6). 4.2000. 0044 oe eee 
Freundlich: 27 202.3: so See ee 1:104:810 
Schulze 6.350 Pe ee ee ek ee ee 1: 49:810 


which only approximate to the Whetham suggestion of 1: x: x’. 

Electrokinetic Effects of Added Electrolytes.?°—Jevons first 
suggested that the coagulating action of electrolytes was due to 
the neutralization of a charge possessed by the particles. Hardy 
found that globulin solutions were most easily coagulated at the 
point where their charge was zero, 7.e., at the time when they 
showed no motion in an electric field (the isoelectric point). 
Following Hardy’s suggestion, experiments were carried out by 
the writer to determine the influence of added electrolytes on the 
mobilities of the particles of gold, silver, and copper Bredig 
solutions. 

Billiter, in making similar experiments on colloidal solutions of 
platinum, mercury, silver, gold, and palladium, to which he 
added gradually increasing amounts of various electrolytes, 
found that the mobility of the particle gradually decreased and 
eventually changed in direction, showing that even the sign of 
the charge was changed by the addition of the electrolyte. He 
added gelatin and urea to his solutions in order to prevent 
coagulation. Whitney and Blake disagreed in toto with the 
conclusions of Billiter, and failed to reproduce his results with 
colloidal solutions of gold and platinum, free from gelatin. They 
assigned Billiter’s change in the direction of migration to the 
dissolved gelatin. 

Exhaustive experiments carried out by the writer on solutions 
of gold, silver, and copper showed conclusively that the addition 
of electrolytes caused a reduction of the mobility of the particles; 
as the mobility approached zero, the coagulation by the electro- 
lyte became more rapid. The amount of reduction in the mobil- 
ity ran directly parallel with the coagulative power of the 


20 Burton: “‘ Physical Properties of Colloidal Solutions,” 2nd ed., p. 163, 
et seq. 


THE KINETICS OF DISPERSE SYSTEMS 139 


electrolyte, which showed that the coagulation was primarily due 
to the discharge of the particles, particularly with divalent and 
trivalent ions as the active coagulants. 

The relation of electrolytes to the mobility of particles is 
exactly the same as the action of the same electrolytes in reducing 
the electroendosmose effects in capillary tubes (see Perrin, 
Elissafof, and Briggs?'). 


MutTvuau AcTION OF COLLOIDAL PARTICLES 


In spite of the opposition expressed by some authors, in the 
opinion of the writer there does exist a mutual action of colloidal 
particles, in a given solution, due to the charge possessed by 
the particles. The fact that a solution containing particles 
charged with one sign, say positive, when added to a solution of 
particles charged with the opposite sign (— ), invariably produces 
mutual coagulation shows that the individual particles can come 
within a region of mutual action. We cannotlook upon the Helm- 
holtz double layer as anything more than a state of affairs brought 
about by the charged particle, whereby we have near the particle 
a slightly increased concentration of ions bearing a charge opposite 
to that on the particle. This ionic atmosphere will be graded off 
to the general concentration in the bulk of the medium. The 
linear extent of this graded ionic atmosphere may be easily of the 
same order as the distance between the particles—a condition which 
would induce a mutual repulsion of the particles. The simplest 
explanation of the uniform distribution of colloidal particles in a 
given sample is the mutual repulsion due to the electric charges. 

There are two very definite proofs that there is a mutual 
action of the particles of a given colloidal solution. In the 
first place, observation with the ultramicroscope will force one 
to the conclusion that the particles do not come into collision 
with one another; two neighboring particles may approach 
and rotate about a common center for a short time, but they will 
always separate before a collision takes place. 

In the second place, the experimental evidence that the 
particles in a given solution are uniformly distributed throughout 
the solution seems quite conclusive (Burton,?? Burton and 


Lococu., nos. 11, 12; 13. 
22 BurTon and Bisuor: J. Phys. Chem. (1921). 


140 COLLOIDAL BEHAVIOR 


Currie?*). Not only so, but for any given material in the 
particles, with given conditions of electrolytic content in the 
medium and given conditions as to gravitational force (including 
that induced by centrifuging), there seems to be a limiting con- 
centration of the disperse phase. In the case of silver sol, the 
writer found that the limiting concentration was such that 
the nearest distance of approach of two silver particles was of 
the order of 50 times the diameter of the particle. On attempt- 
ing to concentrate the sol further, some of the disperse phase was 
forced out of the solution; the amount of electrolyte present in 
the sample was so small that its concentration during the process 
could not account for the change in disperse phase kept in 
suspension. 

The establishing of the existence of this mutual action leads 
to several important inferences. It endows the particle with an 
effective charge capable of reacting with a like charge on neigh- 
boring particles and also capable of inducing motion in an 
electric field, thus offering an explanation of cataphoresis much 
more satisfactory than that given by Helmholtz.” 

More important still is the bearing of this mutual action on the 
application of the osmotic pressure formula, Pv = RT, to these 
solutions. This formula is taken over from the kinetic theory of 
gases and, consequently, this mutual action can be introduced in 
the same manner as one would introduce a correction for a 
mutual action of the molecules in a gas. This mutual action can 
be taken account of by adding to the equation expressing the 
energy of the particles the virial of the mutual forces.**> We have 


WimvV? = 36Pv + 42rF(r) (18) 
where 
= mass of molecule 
velocity of molecule 
= pressure of gas 
= volume of gas 
= distance between two molecules 
F(r) = law of force between two molecules 


eee Ss 
I 


23 BurToN and Currie: To be published later. 

24 Hppmuoirz: Ann. Phys., 7 (1879), 337; Memoirs Lon. Phys. Soc. 
(1888). : 

25 RAYLEIGH: Sci. Papers, 5, no. 304, p. 238. 


THE KINETICS OF DISPERSE SYSTEMS Lad 
Putting 442mV? = RT for one molecular weight, we have for P: 
ae 24RT i 1g BrP (r) (19) 


Whenever the osmotic pressure is treated as a gas pressure, as, for 
example, in deducing the formula for the Brownian movement or in 
Perrin’s distribution law, the value of P will involve not only 
RT as usually used, but also the virial term. 

In the case of the Brownian movement this connection indicates 
that the intensity of the movement depends, to a certain extent, 
on the charge possessed by the particle. A particle which is 
charged and in an atmosphere of similarly charged particles will 
have a greater Brownian movement velocity than if the particles 
were uncharged. It ison record that when electrolytes are added 
to a sample of colloidal solution under the ultramicroscope the 
Brownian movement is reduced even before coagulation sets 
in. 2627 

SUMMARY 


Summarizing the foregoing, we may draw attention to the 
following important kinetic relation of these disperse systems: 

1. The particles are of such a size as to possess a very small 
limiting velocity under gravitation and have supplied most inter- 
esting examples of the application of Stokes’ law. 

2. The Brownian movement affords the best direct optical 
evidence we have of the existence of molecular motions in liquids 
and gases. 

3. The possession of an electric charge by the particles offers 
exceptional opportunity for studying the actions of electrolytic 
ions, both from the point of view of the mobility of the particles 
in an electric field and from the phenomena of coagulation. 

4. The mutual action of the particles, whether from their 
relation to surface tension or electrical repulsion, opens up a very 
promising field of study. 


26 MauttTazos: Ann. chim. Phys., 7 (1894), 559; Compt. Rend., 121 (1895), 
303. 
27 Henri: Bull. Soc. Fr. Phys., 4 (1908), 45. 


CHAPTER VI 
SURFACE ENERGY IN COLLOID SYSTEMS 


By 


WiuuiaAM D. HARKINS 


That heat is taken up by a liquid when it is vaporized at a 
constant temperature is recognized as an every-day phenomenon. 
The heat added is, in general, much greater than that which is 
equivalent to the work done by the vapor in pushing away the 
atmosphere. The vaporization of water into a vacuum at 10°C. 
requires the addition of 10,000 cal. of heat per gram molecule 
of water, or 4.18 X 10'' ergs. Thus, the average energy utilized 
in separating one molecule from the neighboring molecules which 
surround it, when it is inside the liquid phase, amounts to the 
number of ergs per gram molecule, as given above, divided by 
the number of molecules in a gram molecule (6.06 X 10”), or 
69.6 < 107-14 ergs. 

Since energy is thus absorbed in separating a molecule com- 
pletely, as in vaporization, it is natural to conclude that energy 
must be added to a system if some of its molecules are to be partly 
separated from those surrounding them, as when 
a new surface area is formed. That at least a 
part of this energy may be added in the form of 
work is indicated by an arrangement suggested 
by Maxwell (Fig. 1). 

A soap film is stretched between the upper part 
of a wire frame ABCD and a movable cross wire 
EF. If the wire EF is very light, the film will 
contract to a very small area, but it is found 
that, if weighted to a definite weight W, the wire will keep the 
film extended to its initial area. In this case, the tension of the 
two sides of the film along EF is equal to the pull of the weight 

142 





SURFACE ENERGY IN COLLOID SYSTEMS 143 


W.. The surface tension (y) of the liquid is the pull exerted 
by one side of the film along a unit length, or 


y-2d = mg sO y= = (1) 


If the magnitude of the weight necessary to keep the film 
extended is independent of its area, the surface is considered as 
saturated; that is, a saturated film is one whose surface tension is” 
independent of its extension. The surface of any pure liquid is 
saturated if the surface itself is pure. Certain solutions also have 
saturated surfaces, as is practically the case for aqueous solutions 
of sodium oleate at concentrations between 0.01 and 0.1 m. 
The interface between benzol and an aqueous solution of this 
salt is saturated between 0.015 and 0.1 M. 

Though the distinction between saturated and non-saturated 
surfaces is an important one, it is customary to restrict the 
treatment of surface energy relations almost entirely to saturated 
surfaces. Unsaturated surfaces are frequently found on solu- 
tions, and they may occur with a pure liquid if it is spread into a 
sufficiently thin sheet. Recent theory indicates, however, that 
such a sheet must be exceedingly thin—so thin that such films are 
seldom formed. In the treatment which follows, all films will be 
considered as saturated unless otherwise specified. 

For such surfaces, the work of extension is equal to the force 
which produces the extension, multiplied by the distance through 
which the force acts, so the work in ergs necessary to produce a 
unit area of surface is numerically equal to the number of dynes 
which expresses the surface tension. Since this work may be 
entirely given back in the contraction of the surface, this amount 
of energy is said to be present in the surface as free energy. If y 
is the numerical value of the surface tension, then the surface 
tension of the liquid is y dynes per centimeter, and its free surface 
energy is y ergs per centimeter. 

Table I gives the surface tension and the free surface energy of 
a number of liquids at 20°C. 

One of the most important characteristics of surfaces is that 
the surface tension, or the free surface energy, decreases rapidly 
as the temperature increases, and becomes equal to zero at the 
critical temperature. This decrease is often linear, as illustrated 


144 COLLOIDAL BEHAVIOR 


in curve 2 (Fig. 2), although curves of the forms designated as 1 
and 3 often occur. 


TaBLE I.—SurFacr TENSION AND THE FREE SURFACE ENERGY OF A FEW 
TyPpicaL LIQUIDS 























dy 
dy dy = 
Yo [ = 27374 E, S = dt dt 
~ Oe Ne 
Waters 4). Clee ee eros 42.25 118.10 | 0.1511 | 0.00199 
Bromines, aeietean ase OU 144.50 195.50 | 0.5300 | 0.01040 
Sulinvor ss sie 4 Ont 3.82 64.09 | 0.0140 | 0.00023 
Carbon disulfide....... Sy GG Al 43.91 81.60 | 0.1607 | 0.00426 
Meroury Geaate sa el eo eU 60.10 540.40 | ...... 0.00046 
Hexane ie 00) 35 eed Vk 28.15 49.50 | 0.1032 | 0.00484 
Octane yeu eee eee 23 . 36 25.04 48.40 | 0.0920 | 0.00394 
Chioraformeum ati 6 senece 30.94 59.70 | 0.1134 | 0.00394 
Ethyl iodide.......... 33.53 37.51 71.00 | 0.1370 | 0.00409 
Methyl alcohol........| 23.50 19.40 42.90 | 0.0710 | 0.00306 
Ethyl sicaho! gee 23.30 21.70 45.00 | 0.0800 | 0.00343 
Ghycoli nt aa | 49.34 24.52 73.90 | 0.0935 | 0.00189 
Glycerin panne she dace eo ee 16.35 81.60 | 0.0598 | 0.00096 
Benzol ice! .8 Sine cate eee Late 35.64 67.20 | 0.1300 | 0.00413 
Toluene x gee ae 30.76 31.32 62.10 | 0.1150 | 0.00373 
Cvmene. 35 aee 8 eee 30.18 26.57 56.70 | 0.0980 | 0.00323 
Diphenyl: fh eee 24.88 65.30 | 0.0911 | 0.00225 
Phenol (e078 ac eee 42.27 28 .62 70.90 | 0.1050 | 0.00248 





The principle of Le Chatelier indicates that when the state of a 
system in equilibrium is varied, the system changes in such a way 
as to oppose a greater resistance to the change. Thus, if the 
surface is expanded, an increased resistance to further expansion 
would be introduced, provided the surface should become cooler, 
since this would raise the surface tension. From this, it may be 
deduced that a surface cools if it is extended, or in such a case the 
initial temperature may be restored if heat is added. ‘The amount 
of heat required per unit area of extension of the surface is called 
the latent heat! of the surface. 

According to the equation of Clapeyron, first applied to 
surfaces by Lord Kelvin, the latent heat of the surface is given 
by the equation 


p's eee 
ie Sa ae | (2) 


SURFACE ENERGY IN COLLOID SYSTEMS 145 


Thus, the latent heat of the surface is equal to the absolute 
temperature multiplied by the negative of the rate of change of 
the surface tension with the temperature. 

The total energy (wu) of the surface is, therefore, 


u=ytl or u=y— Ton (3) 


Surface Energy 





Temperature Te 


Fic. 2.—The free energy (y) and the total energy (Hs) or (u) of liquids. 


Now, if y is a linear function of the temperature, as it often is, 


a is constant, or the latent heat of the surface is proportional to 
the absolute temperature. In this case, | increases at the same 
rate at which y decreases, or 
u=Il+y7 = constant (3) 

when 7 is a linear function of T’. 

The entropy of the surface S is equal to the latent heat of the 
surface divided by the temperature, or 
Dee Lire ah (4) 


146 COLLOIDAL BEHAVIOR « 


SO 
S = constant 


when ¥ isa linear function of 7’. (4’) 
At low temperatures, the latent heat of the surface is small, and 
the surface energy consists largely of the free surface energy. 
At temperatures nearly as high as the critical temperature, the 
free surface energy becomes very small and the latent heat rela- 
tively large, so in this region the surface energy 1s largely con- 
tributed by the heat motion of the molecules. At the critical 
temperature the surface energy becomes equal to zero, or 


V =.0) at 7 ae 
=) 
u=ytl=0 | Ce 


This indicates that the surface tension (vy) curve becomes 
tangent to the temperature axis at the critical temperature. 
Figure 3 illustrates a molecule A which is in the surface of a 
liquid. This is attracted in all directions except upward, so 
the only entirely unbalanced attraction is that which is down- 
ward. A molecule which rises into the surface against this 
attraction increases its potential energy. At least, the greater 
part, and presumably nearly all, of the surface 

(8) energy is present in this potential form. A 
molecule may rise into such a position against 
this attraction by a utilization of a part of its 
own kinetic energy, but in order that the 
surface area shall be increased by this process, 
the surface must at the same time be expanded 
Lovie by a force which contributes free energy. 
According to this point of view, while the 
energy of the surface is largely potential, it 
may be contributed to the surface by (1) the heat motion of the 
molecules, or (2) work done by forces from the exterior. It is of 
interest to note that, in order that a molecule may get into the 
surface, the average energy contributed from the heat motion of the 
molecules is 144 per cent of the mean energy of translation of a 
gas molecule at that temperature. Thatis, in general, only those 
molecules, the kinetic energy of which is greater than the mean, 
possess enough energy of translation to rise into the surface. 


Vapor 


UKE ae 


SURFACE ENERGY IN COLLOID SYSTEMS 147 


A molecule B of the vapor has a still higher potential energy with 
reference to the interior of the liquid, but the latent heat of 
_ vaporization decreases as the temperature rises. 


WoRrRK AND ENERGY OF SURFACE COHESION 


The relationship between surface energy and cohesion may be 
exhibited in a simple way by the consideration of a liquid bar of 
unit cross-section (Fig. 4). If the bar is pulled apart jean 
along the plane D, all of the work done against the = /sq,cm 
cohesive forces is utilized in the formation of the two | 
unit surfaces at the plane of rupture, or 


W,. = 2y 

Thus, the work done against the cohesive forces is 
equal numerically to twice the free surface energy, 
or, in the case of water at 20°C., to 145.6 ergs. While 
this does not give the value of the force of cohesion, 
some idea of the magnitude of this force may be 
obtained by assuming that the two surfaces exert a 
constant force of attraction until their separation 
reaches 10-§'cm. Now 





W =Fr 
or 
W Fic. 4,— 
F, = — = 14,560 X 10° dynes = 14,400 atmospheres liquid _ bar 
r of unit cross- 
section. 


This calculation could be made more exactly if the functional 
relations between F and r were known, since, if the lower end of 
the bar of liquid (Fig. 4) consists of one liquid and the upper part 
of another, with an interface at D, the work of separation (W4) is 
utilized in producing a unit surface of the liquid 1, and another 
unit surface of the liquid 2, but it is aided by the energy set free 
when unit area of the interface 1, 2 disappears, thus 


Wa =71 7 ¥2 — 471.2 


which is the well-known equation of Dupré. The total energy 


148 COLLOIDAL BEHAVIOR 


of adhesion is given by a simple modification of this equation, as 
given by the writer: 
Ka = (v1 + 11) = (yo Ts ls) a (1,2 te l1,2) 
U1 + Us, —.U1,2 
The value of the work of adhesion (W4) is highly dependent 
upon the chemical nature of the two liquids. Thus, if water is 
one of the liquids, the values of W, are relatively high when the 


other liquid is an alcohol or an acid, and low when it is a hydro- 
carbon or a similar compound, as was found by Hardy in 1913. 


MoNOMOLECULAR FILMS ON THE SURFACE OF LIQUIDS 


That drops of certain liquids spread themselves out into a 
film one molecule thick was pointed out by Lord Rayleigh® in 
1899, and more definitely by Devaux,’ who began his researches on 
films in 1903. This idea has been applied so extensively in con- 
nection with the theory that molecules in surfaces are oriented, 
that the experimental development of the work on films will be 
treated in connection with the orientation theory. 


THe ORIENTATION THEORY OF SURFACE STRUCTURE 


Almost all solid materials possess an internal molecular 
architecture or structure, which consists of an orderly and sym- 
metrical arrangement of its molecules. This structure was first 
made apparent by the symmetrical distribution and orientation 
of the surfaces upon crystals. The interior symmetry of arrange- 
ment was later made evident through the effects of crystals upon 
x-rays as revealed by the fundamental work of Laue. 

The passage of x-rays or of ordinary light through ordinary 
liquids gives no such evidence of any internal structure, though a 
few specific substances are known to exhibit a certain crystalline 
structure when in the liquid state. These are known as liquid 
crystals. 

The most fundamental characteristic of a surface is the unlike- 
ness of its two sides and the resultant dissymmetry of the molec- 
ular forces involved. If the molecules in the surface are not 
entirely symmetrical, this lack of balance in the forces must result 


SURFACE ENERGY IN COLLOID SYSTEMS 149 


in their orientation to a smaller or a greater degree. Since, 
however, the heat motion of the molecules is very great at 
ordinary temperatures, it might well be that the orientation 
would be thus so greatly reduced as to produce no noticeable 
effect upon ordinary surface phenomena. Thus, not only are the 
molecules in the surface of water vibrating with extreme rapidity, 
but their orientation is also disturbed by the escape of about 
7 X 107! molecules from each square centimeter of surface every 
second (at 20°C.). Not only is this the case, but if the water 
and its vapor are in equilibrium, molecules to the same number 
jump back into the surface in the same minute interval of time.* 
It is thus seen that the idea that there are forces which would 
produce orientation in a static system is not sufficient to demon- 
strate that such orientation has an appreciable magnitude. For 
such a demonstration, definite experimental evidence is necessary. 

The idea that the molecules in surfaces are orientated was 
presented in a remarkably clear way by Hardy in 1912 and 1913, 
and this idea was developed into a definite and experimentally 
founded theory by Harkins and by Langmuir independently. 
The principal lines of evidence presented by these two latter 
workers were based upon quite different phenomena, although 
there were naturally many points of similarity, since both investi- 
gators had the benefit of Hardy’s fundamental suggestions. 

The two following quotations, cited from two papers by Hardy, 
include all that he wrote at that time upon the topic of molecular 
orientation. 


* Since there are about 10 molecules of water in 1 sq. cm. of surface, 
this means, if we consider the area covered by one molecule, that 7,000,000 
times during 1 second the molecule in this area at the instant would jump 
out into the vapor, and (also on the average) a molecule would fall from 
the vapor upon this area 7,000,000 times. Since there would also be an 
enormous number of exchanges between the surface of the liquid and the 
molecular layer just below, it will be seen that the surface is in anything 
but a static condition. Thus, if there is to be any appreciable degree of 
orientation of the molecules on the average, the time of orientation should 
fall considerably below 54-000,000 second. Since the data on surface 
energy indicate a marked degree of orientation in most liquids at this 
fraction of their critical temperature (0.437), it would seem that the time 


; ae 1 
of orientation is of the order of 100,000,000 second or less, which seems 


entirely plausible when the rapidity of rotation of such a system is considered. 


150 COLLOIDAL BEHAVIOR 


The corpuscular theory of matter traces all material forces to the 
attraction or repulsion of foci of strain of two opposite types. All 
systems of these foci which have been considered would possess an 
unsymmetrical stray field—equipotential surfaces would not be dis- 
posed about the system in concentric shells. If the stray field of a 
molecule, that is, of a complex of these atomic systems, be unsym- 
metrical, the surface layer of fluids and solids, which are close-packed 
states of matter, must differ from the interior mass in the orientation 
of the axes of the fields with respect to the normal to the surface, and 
so form a skin on the surface of a pure substance having all the 
molecules oriented in the same way instead of purely in random ways. 
The result would be the polarization of the surface, and the surface of 
two different fluids would attract or repel one another according to the 
sign of their surfaces. (Hardy, 1912.) 


These ideas are even more clearly expressed in the following 
passage. 


If the field of force about a molecule be not symmetrical, that is to 
say, if the equipotential surfaces do not form spheres about the center 
of mass, the arrangement of the molecules of a pure fluid must be 
different at the surface from the purely random distribution which 
obtains on the average in the interior. The inwardly directed attractive 
force along the normal to the surface will orientate the molecules there. 
The surface film must, therefore, have a characteristic molecular 
architecture, and the condition of minimal potential involves two 
terms—one relating to the variation in density, the other to the orienta- 
tion of the fields of force. (Hardy, 1913.) 


While it is not to be expected that the surfaces of two fluids 
will in any case repel each other unless the surfaces are charged 
in addition to the polarization produced by orientation, it is seen 
that in these paragraphs Hardy states the idea of orientation 
quite definitely, and in the latter paragraph calls attention to | 
the important principle of the minimum potential. 

This principle was also used in Langmuir’s important contri- 
butions to this subject. His initial ideas upon this subject are 
presented below in the form of quotations from his first paper on 
orientation.'! 

1. According to this theory, the group molecules of organic liquids 
arrange themselves in the surface layer in such a way that their active 
portions are drawn inwards, leaving the least active portion of the mole- 
cule to form the surface layer. 


SURFACE ENERGY IN COLLOID SYSTEMS 151 


2. Surface tension (or surface energy) is thus a measure of the poten- 
tial energy of the electromagnetic stray field which extends out from 
the surface layer of atoms. The molecules in the surface layer of the 
liquid arrange themselves so that this stray field is a minimum. 

3. The surface energy of a liquid is thus not a property of the group 
molecules, but depends only on the least active portions of the molecules 
and on the manner in which these are able to arrange themselves in the 
surface layer. 

4. In liquid hydrocarbons of the paraffin series, the molecules arrange 
themselves so that the methyl groups (CHs;) at the ends of the hydro- 
carbon chains form the surface layer. The surface layer is thus the 
same, no matter how long the hydrocarbon chain may be. As a matter 
of fact, the surface energy of all these many different substances, from 
hexane to molten paraffin, have substantially the same surface energy— 
namely, 46 to 48 ergs per square centimeter, although the molecular 
weights differ very greatly. 

5. If, now, we consider the alcohols, such as CH;OH, C.H;OH, etc., 
we find that their surface energies are practically identical with those of 
the hydrocarbons. The reason for this is that the surface layer in 
both cases consists of CH; groups. 

6. With such substances as CH;NO:s, CHsI, we find that the surface 
energy is much greater than that of the hydrocarbons. This is due 
to the fact that the volume of the I or the NO, is so great that the 
surface cannot be completely covered by the CH; radicals. The forec- 
ing apart of these groups increases the surface energy. 

7. In benzol itself, the group molecules arrange themselves so that 
the benzol rings lie flat on the surface, since the flat sides of these rings 
are the less active portions of the molecules. The surface energy of 
benzol is about 65 ergs per square centimeter. 

8. If, now, an active group, such as OH, is substituted for one of the 
hydrogens in the benzol (forming phenol or carbolic acid), this group is 
drawn into the body of the liquid, tilting the benzol ring up on edge and 
raising the surface energy to about 75 ergs per square centimeter, which 
corresponds to the activity of the perimeter of the benzol ring. Thus, 
any active group strong enough to tilt the ring up on edge raises the 
surface energy to about 75. Two active groups side by side (ortho 
position) have no greater effect than one. But two active groups oppo- 
site one another (para position) cannot both go wholly below the surface, 
so that the surface energy then becomes abnormally large (about 85 
in the case of paranitrophenol). The substitution of methyl or ethyl 
groups in the benzol ring lowers the surface energy, except where an 
active group in an adjacent position draws these groups below the 
surface. 


152 COLLOIDAL BEHAVIOR 


9. Some of the best evidence in support of the new theory is derived 
from experiments on thin films of oil on water or mercury. Oleic acid 
on water forms a film one molecule deep, in which the hydrocarbon 
chains stand vertically on the water surface with the COOH groups in 
contact with the water. 

10. Acetic acid is readily soluble in water because the COOH group 
has a strong secondary valence by which it combines with water. 
Oleic acid is not soluble because the affinity of the hydrocarbon chains 
for water is less than their affinity for each other. When oleic acid is 
placed on water, the acid spreads upon the water, because by so doing 
the COOH can dissolve in the water without separating the hydro- 
carbon chains from each other. SEE 

11. When the surface on which the acid spreads is sufficiently large, 
the double bond in the hydrocarbon chain is also drawn down onto the 
water surface, so that the area occupied is much greater than in the case 
of the saturated fatty acids. 

12. Oils which do not contain active groups, as, for example, pure 
paraffin oil, do not spread upon the surface of water. 

13. The measurement of the area of water or mercury which can be 
completely covered by a given amount of a substance affords an accurate 
method of determining the shapes of group molecules. Thus it is found 
that the molecules of stearic acid on a surface of water have a length of 
about 23 X 1078 cm. and cover an area of 24 X 10716 sq. cm. ‘These 
measurements prove that the molecules are not spherical, but are much 
elongated. 

An independent development of the orientation theory by 
Harkins and his associates arose from considerations presented 
for many years in the lectures of Prof. Julius Stieglitz at the Uni- 
versity of Chicago. In an application of the principle “like dis- 
solves like,’’ he emphasized that the carboxyl group of an organic 
acid gives the acid its solubility in water, while the hydrocarbon 
chain contributes to its solubility in an organic phase. The 
writer had been working upon a two-phase system consisting of 
water and benzol, and it occurred to him that any butyric acid 
dissolved in the two phases should, when equilibrium ts attained, 
reach by far its highest concentration at the interface, since there 
the hydrocarbon group could dissolve in the organic phase, and the 
carboxyl group in the water. 

While the primary attention of the work in the Chicago labora-. 
tory has been given to orientation at interfaces, it was natural 
that the subject of films of oil on water should also be considered. 


SURFACE ENERGY IN COLLOID SYSTEMS 153 


Thus, in considering the spreading of oleic acid on water, the 
writer expressed the fundamental idea ‘‘COOH of acid down 
because both acid and HO associated and polar,’”’ as is shown by 
_Fig. 5, which reproduces a section from the lecture notes* 


COOH of Acid down because both 
Acid and H20 associated and polar 


(A) 





If Te>la+/aB spread 
Ta~<Ta+la8 notspread 


Te = 72.8 I 
Tas 34.3 2 (F) 
= 2888 | tee 


Here 72. ie >63./8 .. Drop of oa will 
spread over Water 


10), (B) (E) 


Fie. 5.—A reproduction of notes showing the essential basis of (1) the 
theory of the orientation of molecules in surfaces, and (2) the principle in- 
volved in the spreading of one liquid over the surface of another. (A) which 
states ‘‘COOH of acid down because both acid and water associated and polar”’ 
is a brief statement of the present theory of orientation. (#). illustrates the 
lowering of the surface tension of water by a film of oriented molecules of oleic 
acid. (C) exhibits the rise of water in a capillary tube covered above by a 
benzol phase. (JD) illustrates the same when the upper phase is water vapor, or 
vapor and air. (#) represents a drop of oil which does not spread on water. 
(F) gives the principle of the Neumann triangle, and shows that according to this 
principle benzol will spread on water, even although it contains no very polar 
group (hexane also spreads). The notes were taken by George L. Clark from a 
lecture by the writer as delivered in March, 1914. They represent portions 
selected from a single page of the note-book. 





of George L. Clark, taken in March, 1914. This is the earliest 
record to give the actual orientation of the molecules in any 
surface. 

Definite evidence that, notwithstanding ne heat motion of the 
molecules, there is an actual mean orientation of the molecules at 
an interface or a surface was obtained by Harkins, Brown and 
Davies!” by a comparison of the work necessary to pull apart a 
unit bar of a pure liquid W,—and that used in separating a unit 

* These notes state that benzol spreads on water, a fact noted earlier 


by Hardy. Attention is called to this point, since certain later writers have 
claimed that benzol does not spread. 


154 COLLOIDAL BEHAVIOR 


bar with two unlike ends just at the interface between the two 
liquids Wy. Molecules of the type of those of the paraffins may 
be designated as slightly polar or homopolar, while groups of the 
nature of COOH, etc., may be styled polar. A molecule such as . 
that of butyric or lauric acid may be designated, therefore, as 
polar-homopolar. Such molecules have been represented in this 
laboratory for many years by the symbol J, in which the upper 
rectangular part represents the homopolar hydrocarbon chain, 


Water 











OR sy Se 
OWA 
C) QS bp C) 
= ee. 
om KI) 










Bs 


~S 
{) 
K Ns 


M4 
LTS 


Oriented Wedge(or Truncated 
Cone) Theory of Emulsions 







Usd, 


Ow 


SY 


Fic. 6.—Molecules with ends weighted toward water. (Drawing by Ernest B. 
: Keith). 

and the lower circle, the polar group. In the form of a model, 
the upper part is a cylinder made of wood, while the lower part, 
which designates the polar group, is made of iron. Such models 
may easily be so constructed that they float upright upon the 
surface of a body of water, with the tops of the wooden cylinders 
projecting above the surface of the water. A crowded assemblage 
of such models represents very well the general configuration of 
a layer of an organic alcohol or acid upon the surface of water, 
except that the models are much less flexible than the molecules, 
and motion corresponding to the molecular motion is absent. 
Figure 6, which will be referred to later in connection with the 
subject of emulsions, gives a highly conventionalized set of draw- 
ings to illustrate the behavior of models with an end weighted 


SURFACE ENERGY IN COLLOID SYSTEMS 155 


toward water in the case of drops of oil in water, and of water in 
oil. In subfigures 1 and 5 the polar group is represented by 
cross-hatching, and in the other diagrams by circles. It is not 
to be supposed that the orientation is so perfect as this in an 
actual case, except at very low temperatures. 

Subfigure 6 (Fig. 6) gives also the orientation which results 
if a drop of a polar-homopolar oil, such as nonylic or butyric 
acid, is suspended in air. The general principles involved are 
presented in the following quotation from a paper by Harkins, 
Davies and Clark:' 


1. The molecules in the surfaces of liquids seem to be oriented, and in 
such a way that the least active or least polar groups are oriented toward 
the vapor phase. ‘The general law for surfaces seems to be as follows: 
If we suppose the structure of the surface of a liquid to be at first the same 
as that of the intervor of the liquid, then the actual surface 1s always formed 
by the orientation of the least active portion of the molecule toward the 
vapor phase, AND AT ANY SURFACE OR INTERFACE THE CHANGE WHICH 
OCCURS IS SUCH AS TO MAKE THE TRANSITION TO THE ADJACENT PHASE 
LESS ABRUPT. This last statement expresses a general law, of which 
the adsorption law is only a special case. If the molecules are mon- 
atomic, and symmetrical, then the orientation will consist in a dis- 
placement of the electromagnetic fields of the atom. This molecular 
orientation sets up what is commonly called a ‘‘double electrical layer” 
at the surfaces of liquids and also of solids. 

This law, if applied to special cases, indicates for a few pure liquids the 
following orientation: In water the hydrogen atoms turn toward the 
vapor phase and the oxygen atoms toward the liquid. With organic 
paraffin derivatives, the CH; groups turn outward, and the more active 
groups, such as NO2, CN, COOH, COOM, COOR, NH2, NHCHs:, NCS, 
COR, CHO, I, OH, or groups which contain N, 8, O, I, or double bonds, 
turn toward the interior of the liquid. 

If any of these organic compounds are dissolved in water, their 
orientation in the water surface is the same as that just given, with the 
active groups inward. | 

At interfaces between two pure liquids, the molecules turn so that 
their like parts come together in conformity with the general law. With 
solutions, the solute molecules orient so that the ends of the molecules 
toward the liquid A are as much like A as possible, and the ends toward 
B are as much like B as possible. So at interfaces between organic 
liquids and water, for example, the organic radical sets toward the 
organic liquid, and the polar group toward the water. 


156 COLLOIDAL BEHAVIOR 


2. If at an interface the transition from a liquid A to the liquid B is 
made by a saturated film of solute molecules which we may call A-B, that 
is, they have one end like A and the other like B, then the free surface 
energy is greatly reduced. For example, with water and benzene with 
sodium oleate as the solute, the free energy falls as low as 2 ergs per 
square centimeter. 

3. If the solvent is polar, such as water, then solutes will, in general, be 
positively adsorbed in the surface if they are less polar than water, and 
the least polar end of the molecule will be turned outward. Solutes 
more polar than water are negatively adsorbed. 

4, The stability of emulsoid particles seems to be brought about by 
orientation of molecules at the interface with the medium of dispersion. 
The best emulsifying agents, for example, have very long molecules, 
with a polar or active group at one end of the molecule. For the emul- 
soid particle to be stable, the molecules which make the transition from 
the interior of the drop to the dispersion medium, or the molecules of 
the ‘‘film,” should fit the curvature of the drop (Fig. 6). 

From this standpoint the surface tension of very small drops is a func- 
tion of the curvature of the surface. 


Definite evidence in support of the general principle of orienta- 
tion, as expressed in paragaph.1 above, is contained in the data 
(Harkins, Brown and Davies) for the work done in pulling apart 
a liquid bar of unit cross-section (Fig. 4). Water is a highly 
polar compound, and it is found that the work W, required to 
pull a unit bar of water into two parts in such a way that two 
surfaces of unit area are created is 145.8 ergs, a relatively large 
number as compared to the value given by an organic substance. 
The work required to accomplish the same result for the slightly 
polar compound octane is less than a third of that for water, 
that is, for octane the value is only 43.5 ergs. 

It is of interest to determine how the attractive forces between 
octane and water are affected by the polar nature of the water. 
The surprising result expressed by the data is that it requires 
practically no more work to separate octane from water at the 
interface between the two, than it does to separate octane from 
octane. Thus, the value of the work of adhesion between octane 
and water is 43.8 ergs, identical, within the limits of error, with 
that found for the work of cohesion of octane. 

When a bar of octyl alcohol itself is pulled apart, the first effect 
to be expected is that, where the break is to occur, the molecules 


SURFACE ENERGY IN COLLOID SYSTEMS 157 


on both sides of the plane of the break should orient themselves so 
that the break would occur with the least possible expenditure of 
work. Thus, the molecules should first orient themselves so that 
the final break can occur between the ends of hydrocarbon chains 
(Fig. 7). From this point of view, 
the final amount of work of rupture 
should be not very different from 
that given above for octane, or 
43.5 ergs. However, two factors 
should increase this value some- 
what: First, at the very beginning 
of the process which results finally 
in the separation, a moderate 
number of hydroxyl groups, which 
will finally lie on one side of the 
plane, must be separated from VAPOR 
others which will finally lie on the 
other; and, second, the heat motion 
of the molecules should detract 
somewhat from the perfectness of 
the orientation. When these two 
factors are taken into consideration, 
it is seen that the moderate increase 
in the value of the work to 55.1 ergs 
is not surprising. 

When, however, octyl alcohol is 
pulled away from water, polar Fie. 7.—Orientation produced 

when a bar of oleic acid is pulled 
hydroxyl groups must be pulled snare. 
away from polar water, so a high 
value of the work of adhesion should result from the orientation at 
such an interface. Corresponding with this it is found that the 
work of adhesion between octyl alcohol and water (91.8 ergs) is 
very much greater than the work required to pull octyl alcohol 
from octyl alcohol (55.1 ergs). 

Even more convincing than the above is the fact that the work 
required to pull alcohol away from water is nearly independent of 
the size of the molecule, that is, of the fraction of the molecule in 
the hydrocarbon chain, which would be an entirely unexpected 
result if orientation is not assumed. Thus, an increase of the 








158 COLLOIDAL BEHAVIOR 


hydrocarbon chain from 1 carbon atom in methyl alcohol to 8 
carbon atoms in octyl alcohol reduces the work of adhesion only 
from 95.5 to 91.8 ergs. 

A consideration of the relations at the interface between octyl 
alcohol and water, from the standpoint of the dimensions found 
for alcohol molecules in films on water, is of interest. It will be 
seen later that the number of alcohol molecules per square cen- 
timeter is about 3 X 1014, while it is easily calculated that for 
symmetrical water molecules the number is about 10 x 10”. 
A simple calculation shows that the work necessary to pull the 
alcohol from water at the interface between the two is about 
30 X 10- ergs, while in separating water from water it is about 
15 X 10- ergs, per molecule of water on one side and in a plane. 
These energy values seem to indicate the probability that at 
the interface the —OH groups of the alcohol are adjacent to 
several molecules of water. Also, they suggest that the fact 
that the work of separation of the alcohol from water (91.8) 
is smaller than that of water from water (145.8) is not due to 
the relative smallness of the attractions around the hydroxyl 
group of the alcohol, but to the relatively small number of 
such groups as: compared with the number in the surface of 
water, the ratio being only about I to 3.3. 

The molecule of octane contains 26 atoms. The introduction 
of a single oxygen atom into this molecule increases the work of 
surface cohesion in water by only 26 per cent, but it more than 
doubles the work of adhesion, actually increasing the value by 
111 per cent, which is a remarkably high effect for the addition 
of a single atom per molecule. The values for capryllic acid, 
with 8 carbon atoms, are almost identical with those for octyl 
alcohol, since the work of cohesion for capryllic acid is 57.6 
ergs, while its work of adhesion toward water is 93.7 ergs per 
square centimeter. Thus it will be seen that in the case of non- 
symmetrical molecules, such as those of octyl alcohol (CgHi7- 
OH), capyrllic acid (C7H:;COOH), and mercaptan (C2H;SH), 
the adhesional work Wis determined by the strongest electromagnetic 
fields in the molecule, while the tensile-free energy W. 1s determined 
by the weakest fields, so for unsymmetrical molecules the work of 
adhesion is relatively high, and the work of cohesion low. In 
the case of entirely symmetrical molecules there could be no 


SURFACE ENERGY IN COLLOID SYSTEMS 159 


orientation, though a molecule which is symmetrical in the gase- 
ous state may be expected to become less symmetrical when 
placed in the non-uniform electrical field at the surface of a liquid. 
An increase in symmetry, without a change in the composition of the 
molecule, 1s found to increase the work of cohesion, and to decrease 
the work of adhesion toward water, which is exactly in accord with 
the hypothesis that the molecules in surfaces are oriented, since an 
increase of symmetry not only reduces the extent of the orienta- 
tion, but it also decreases the effect of such an orientation upon 
the energy values. ‘Thus it is only when an organic molecule is 
moderately symmetrical with respect to the electromagnetic 
field (largely electrical) which it produces that the work of 
cohesion can become greater than that of adhesion. 

It is thus found that the value of W4 — Wc, which will be desig- 
nated as S, is dependent upon lack of molecular symmetry for its 
high positive values, and upon the presence of such symmetry for 
its high negative values. For the highly unsymmetrical alcohols, 
S is about 50, while for the symmetrical acetylene tetrabromide 
it is —5.7, and for methylene iodide (CHel.) it is —26.5. It will 
be shown later that S is an important function in connection with 
spreading, so it may be called the spreading coefficient. In 
general, liquids will spread when S is positive, and will not spread 
when S is negative. 

Figure 8 gives the adhesional work for a number of different 
liquids. Let us consider carbon disulfide and ethyl mercaptan. 
The cohesional work in the former is much higher, 62.8 instead of 
43.6, yet the attraction between water and carbon disulfide 
(adhesional work = 55.8) is much less than that between water 
and mercaptan (68.5). 

The former is a symmetrical molecule, and the latter is unsym- 
_ metrical. The hydrosulfide group is evidently more polar than 
the divalent sulfur atom, but when the mercaptan lies in contact 
with the water, most of the hydrosulfide groups are turned toward 
the water, and when they are pulled from it, the polarity of the 
group is evident in the high value of the adhesional work. The 
= § group, not being so polar, gives a considerably smaller value, 
which is 12.7 ergs less. However, the attraction between the 
sulfur of carbon disulfide and water, and also that between the 
sulfur in the different molecules of the carbon disulfide itself, is 


160 


Adhesional Work 


COLLOIDAL BEHAVIOR 












Re om 












C 
EthuloropylhetrrO2S: Octyl Alcohol 
Isovaleronstrr/ “A | | 
Dj isobutyl amnre 5-Octy! Alcohol 





Acetylene tetrabromide 







Ethyl ether 
ofthyl nonylate 






14 








Isobbty! istide 















er tiary butyl chloride 
ge oandimXylene SS Chloroform 
Ethyl bromide 






Ethy/ benzene 






ete ss 
24 eon derane : 
Q 10 20 50 40 50 60 
Temperature, °C 


Fig. 8.—Adhesional work, ergs per sq. cm., between organic liquids and water. 
(The names of the substances represented by the curves are given at the right 
while the names given in the middle of the diagram represent substances for 
which the values are given at 20° only.) It will be seen that the substances 
with symmetrical molecules are near the bottom, and those with unsymmetrical 


molecules near the top of the illustration. 


SURFACE ENERGY IN COLLOID SYSTEMS 161 


‘much greater than the attraction between hydrocarbon groups 
such as C2.H;—. Now when a bar of carbon disulfide is pulled 
apart to make two surfaces, sulfur must be pulled away from 
sulfur, so the cohesional work and also the total cohesional energy 
are relatively high, the former having a value of 62.76 ergs per 
square centimeter. However, when mercaptan is pulled apart, 
sulfur (—SH) does not need to be pulled from sulfur, but the 
sulfur turns under the surface, and only the hydrocarbon groups 
have to be pulled apart, so the work of separation is low (only 
43.6). 

A comparison of the halogen derivatives is also instructive. 
The cohesional work for carbon tetrachloride, chloroform, and 
methylene chloride is almost the same (53.32, 54.26, 53.04), but 
the adhesional work toward water rapidly increases in the order 
given (56.16, 67.30, 71.0). Here the increasing polarity is not in 
evidence in the cohesional but is present in the adhesional work, 
since, when the pure liquids are pulled apart, the increase of 
cohesional work due to an increase of polarity is counterbalanced 
by the concomitant increase of dissymmetry, which allows the 
less polar parts of the molecules to be oriented into the surface. 
At the interface, however, it is the most polar part which is 
turned into the interface, so the effects add together instead of 
subtracting. Also, the adhesional work for isobutyl and tertiary 
butyl chloride are practically as high as in the case of methylene 
chloride, since the chlorine is turned toward the water, but the 
cohesional surface work drops to very low values, 43.88 and 39.18 
ergs per square centimeter. 

Both carbon tetrachloride and ethylene dibromide give the same 
value for the cohesional work as for the adhesional work, but, as 
the number of bromine atoms in the compound increases (acety- 
lene tetrabromide), the cohesional work becomes the higher. 
These compounds have very symmetrical molecules. 

A comparison of isopentene with trimethyl ethylene and of 
octane with octylene shows that the introduction of a double 
bond increases the cohesional work very slightly and the adhe- 
sional work very greatly, especially in the latter case, where the 
double bond is at the end of the molecule. These facts are again 
exactly in accord with the orientation theory. For octane the 

cohesional work is 43,54, while for octylene it is almost the same, 


162 COLLOIDAL BEHAVIOR 


or 44.66, so the introduction of the double bond has little effect. 
The value of the adhesional work for octane is practically 
the same as that for the cohesional work, but the addition of the 
double bond in octylene raises the value by about 60 per cent. 

In a later paper (Harkins and Cheng!*) it is shown that the 
total adhesional energy and the total cohesional energy exhibit, 
in general, exactly the same relations as those given above for 
the work involved, except that in the former case all of the 
energy values are greater. Thus, the addition of 1 oxygen atom 
to the 26 atoms already present in octane to form octyl alcohol 
increases the cohesional energy by only 2 per cent, but the 
adhesional energy by 65 per cent. The cohesional energy of 
ethylene dibromide is, on account of the symmetry of the mole- 
cule, much greater than that of the isomeric but unsymmetrical 
ethylidene dibromide. 7 

The general effect of double bonds near the end of the molecule 
is to increase the adhesional, but not the cohesional, work. 
The double bonds in benzol are distributed with such symmetry 
that they increase both the cohesional and the adhesional work. 


ORIENTATION OF MOLECULES IN SURFACES AS SHOWN BY THE 
Tora, SuRFACE ENERGY AND THE HpAT OF VAPORIZATION 


Remarkable evidence for the orientation of molecules in surfaces 
was obtained several years later (Harkins and Roberts,'4 1921) 
by considering the average amount of energy necessary to raise & 
molecule from the interior of a liquid into the surface e, with that 
necessary to cause it to jump out from the surface 7, the energy of 
thermal emission. The sum of these two equals the heat of 
vaporization \, ore + 7 =X. 

It is obvious that with unsymmetrical (polar-homopolar) 
molecules, represented by the symbol {, all that has to be 
done in getting the molecule into the surface is, according to 
the orientation theory, to lift its (electromagnetically) light 
end into the surface, while, when the molecule jumps out from 
the surface, the heavy end has to be lifted, so e is small 
as compared with j, or with 4. However, with a symmetrical 
molecule there is no (electromagnetically) relatively light end, 
so e is much larger as compared with j or \. Thus, if we plot the 


SURFACE ENERGY IN COLLOID SYSTEMS 163 


€ 
=] 
r 
higher and higher as the molecules become more symmetrical. 
Remarkably in accord with this prediction, Fig. 9 shows that this 


ratio ~ the orientation theory tells us that the curves should lie 


ade 
i 


Ratio of molecular total surface energy to internal molecular heat of vaporization ( 


So 





Corresponding Temperature 


Fie. 9.—The curves show that the ratio 5 of the surface energy (e) to the heat 


of vaporization (A) increases as the symmetry of the molecule increases, as 
corresponds with the theory that the molecules in surfaces are oriented with the 


electromagnetically “‘lightest’’ end up. 


is exactly the case, for the most unsymmetrical molecules, 
those of ethyl and methyl alcohol, lie by far the lowest, while 
symmetrical molecules, such as nitrogen and oxygen, lie very 


164 COLLOIDAL BEHAVIOR 


much higher in the plot, with mercury, which is monatomic, the 
highest. Furthermore, the curve for the highly symmetrical 
CCl, lies higher than for any of the other organic substances, and 
it will be seen that the height of the curve for any organic sub- 
stance is in complete accord with its degree of symmetry. 

It is of interest to note that the value of the surface energy 
becomes a larger fraction of the (internal) energy of vaporization 
as the corresponding temperature increases. The shape of the 
curves seems to indicate that at the critical temperature the 
ratio becomes 1, or the total surface energy is equal to the heat 
of vaporization, that is, a molecule which is in the surface is 
already vaporized. 


ORIENTATION AND MONOMOLECULAR FILMS 


When a small amount of oil, of the general nature of olive oil, 
is put upon the surface of water, the surface tension of the water 
is not affected, provided the area is great enough. The oil 
spreads out until a definite area has been covered, and, in general, 
shows no tendency to spread further. In 1891 Miss A. Pockels?® 
showed that, as the area of the surface is decreased by means of 
movable barriers, a moderately sharp limit is reached at which, 
upon further decrease in area, the surface tension begins to 
decrease rapidly. These experiments were repeated by Lord 
Rayleigh,© who determined the surface tension for films of 
different mean thicknesses. His results for films of castor oil are 
shown in Fig. 10, which indicates that the minimum thickness of 
the film which affects the surface tension of water is 13 X 107° 
em., while for olive oil he found it to be 10 X 10~* cm. Since an 
ordinary atom has a diameter of the order of 2 X 10~§ cm., these 
films have a thickness of about 5 ordinary atoms, but it will be 
found that the atoms of carbon are spaced more closely than this. 
Rayleigh (1899) came to the following conclusion: ‘“‘ We conclude 
that the first drop in tension corresponds to a complete layer one 
molecule thick, and that the diameter of a molecule of oil is 
abOUp aL LO Citys 

However, he considered that the film thickened, until at the 
point C the layer was 2 molecules thick. Devaux’ studied thin oil 
films much more extensively. Hesays: ‘‘ We know, therefore, that 
a film of oil at its maximum extension is formed of only a single 


SURFACE ENERGY IN COLLOID SYSTEMS 165 


layer of molecules.” He considers that the lowering of surface 
tension between B and C, Fig. 10, is caused by a closer packing 
in the monomolecular film, which is the present view. These 
researches established the existence of a two-dimensional region 
of matter in a novel way, although the ordinary surface energy 
relations of liquids also indicate the existence of a surface region 
which has different characteristics from the interior of the 
liquid phase. 








—E 

Oo 

= 

tev) 

a ia N 

r 30 Het tt 


(ee a 
10 
le 


090-20 30 40 50 GO 10 BO 50 
Thickness tn Angstrom Units 


Fig. 10.—Oil films on water. Variation of surface tension with thickness. 
(Rayleigh). 


In the work of Miss Pockels,* and of Rayleigh® and Devaux,’ a 
tray of the nature of a photographic tray was used to contain the 
water. The surface was purified by sweeping with strips of 
paper or of glass, and the film was confined by barriers of the same 
materials. Barriers of glass have the advantage that their weight 
holds them in place, and their use is simple if the surface of the 
water is given the shape of a great convex meniscus rising above 
the sides of the trough. Lord Rayleigh determined the tension of 
the surface by Wilhelmy’s method, that is, by measuring the pull 
upon a knife blade suspended from a balance in such a way that 
only the lower edge of the blade touches the film. Langmuir! 
introduced an ingenious modification of this method, since he 
placed the blade in a horizontal position, making it so thin that 
it would float, and used it in place of the movable barrier at one 
end of the film. The float was suspended from a balance in order 
to measure the force of displacement. Since one side of the 


166 COLLOIDAL BEHAVIOR 


float A is kept in contact with a pure water surface, and the other 
side with the oil film, the balance determines the difference 
between the surface tension of water and that of the film, which is 
sometimes designated as the force of compression. The method 
gives the same results as that used by Rayleigh, but the accuracy 
is increased, since the uncertainty as to the angle of contact of 
the film with the blade is removed. 

The orientation theory indicates that the molecules of the oil 
would be oriented in such a way that the groups most strongly 
attracted by the water turn toward the aqueous phase. From 
chemical evidence Langmuir concludes that these are the polar 
groups, and this is proved even more directly by the direct 
measurements by Harkins and his collaborators of the energy 
values involved, in particular the work of adhesion. 

By a combination of the ideas of a monomolecular film and 
that of orientation with the knowledge of the number of molecules 
in a gram molecule, Langmuir!> was able to determine the mean 
dimensions of the spaces occupied by the molecules in the film, 
upon the basis of asimple assumption. T his was that the density 
of the oil in the film is the same as that of the same oil in bulk. 
While it seems certain that this is not strictly true, the method 
seems to be the best thus far used for the determination of the 
dimensions of non-spherical molecules. The interesting nature 
of the results obtained is demonstrated by the following table. 




















‘TABLE SLL 
Cross- Sa Length 
Substance Formula section Nae Length} per 

A.Us onoue cation 
Palmitic acid Cia oeeeece las C,;H;,;COOH ae 4.6 24.0 1.50 
Stearic acid mR ee oa C17H;,; COOH pep 4.7 25.0 1.39 
Cerotic acidi.o. ss C.o;H;,COOH 25 5.0 31.0 L260 
Tristearin Be ec rr ee (CisH35O02)3C3Hs 66 8 1 25.0 1.02 
Oleic’ avid 0s se C1,H3;;COOH 46 6.8 11.2 0.62 
Myrieyl alcohol..... C30Hs.0H ry | 5.2 41.0 1.37 


ee 
* The values for this acid are incorrect. 


It is apparent that the mean area per molecule does not increase 
very rapidly with the length of the chain in a normal acid, but is 


SURFACE ENERGY IN COLLOID SYSTEMS 167 


greatly increased as the number of chains in the molecule increases. 
These results are in complete agreement with the theory of molec- 
ular orientation. The value given above for oleic acid is approxi- 
mately twice that given by later work in the case of a condensed 
film, and is apparently due to the appearance of what is desig- 
nated as an expanded film. 


i | 
hse Saturated acid +-Il- Saturated acid —WV-4008 Oleic 
(on freshadlstilled ondilute HCI / atidon 


Dynes per cm. 


Farias kd 

1s A 

OS Nec SE 

0 a See 

ip neVecescO 22 aa 7 20 22 = 16 2 VouLOurLl C4 10 (eGo Se 
Areas per molecule in Angstrom Units 





Fig. 11.—Force of compression for films of oil. 


The investigation of thin films on water has recently been 
extended in. a series of measurements by N. K. Adam,'® who 
introduced refinements in the experimental methods, and exer- 
cised great care in the determinations. (Highly accurate meas- 
urements of this type have been made also by Harkins and 
Morgan). Figure 11 presents a few of his results upon the 
areas per molecule with films of palmitic acid, af oleic acid, 
and stearic nitrile, at different compressions. Since the com- 
pression is merely the surface tension of pure water minus 
that of the film on water, it is seen that the base of this 
figure represents, at 20°, a surface tension of 72.8 dynes per 
centimeter. When the compression rises to 54 dynes per centi- 
meter before it collapses or buckles, as occurs at J, H with satu- 
rated acid on old distilled water, this indicates that the surface 
tension of the oil film has been reduced to 18.8 before the collapse 
occurs upon further compression. A collapse or buckling seems 


168 COLLOIDAL BEHAVIOR 


to be due to a heaping up of the molecules, commonly along 
lines nearly parallel to the barrier, but actually with the approxi- 
mate form of the arcs of circles. These curves are all obtained by 
decreasing the area of the film by moving a barrier of glass, which 
iies on the top edges of the tray, closer and closer to the floating 
barrier, which is attached to the beam of the balance, and thus 
measures the force of compression. Adam considers that an 
extrapolation of the line HG to the base of the figure, as in curves 
I, II, or III, gives the area of the hydrocarbon chains, while an 


: OE ee ee 
30 
ERASED [MYRISTIC ACID] | he 








Dynes perm. 


on 
Onl 
oO 
ol 
O1 
Sy 
oOo 


0 
20 25 30 35 40 4 
Areas per Molecule, A°U 


Fig. 12.—Expanded films of myristic acid at 3.5° and 28° on 0.1 n HCl, the 
remainder on 0.01 n HCl. 


extrapolation of the straight line GF in curve III gives the area 
occupied by the head of the molecule, that is, of the carboxyl 
group, supposed to be in contact with the water. (Films such as 
those illustrated by curves I or II are considered as ‘‘ condensed 
films.’’) 

Earlier investigators had found that condensed films may have 
the characteristics of a solid or of a liquid. The two may be 
distinguished by dropping an extremely fine and light wire upon 
the surface. Gentle blowing is followed by a free movement of 
such an object if the surface is of the ‘‘liquid,”’ but not if of the 
“solid,” type. Labrouste® showed that films not only “melt,” 
but they also evaporate in two dimensions. ‘That is, a rise of 


SURFACE ENERGY IN COLLOID SYSTEMS 169 


temperature causes the molecules to move in the two-dimensional 
surface in a manner analogous to that in gases, provided the 
two-dimensional pressure, known as the ‘‘compression,” is kept 
sufficiently low. Figure 12 gives Adam’s results upon the 
“‘gaseous”’ or expanded films obtained with myristic acid. At 
3.5° the film does not become expanded at any compression 
given. It will be seen that each expanded film is transformed 
into a condensed state at a sufficiently high compression. At 


50 







m 
> 
L@n] 


i 
cS 










ater ACID 


Seg ee 
_. Saas 
_ SS See 


Seon coe es 50° 35> 40) 45 50 55 60 65° 10 
Temperature 


[en] 
ot 





nm oO 
Clerc 


Area at 1.4 dynes perc 





fae) 
Oo 


Fig. 13.—Transition from condensed films at lower temperatures to expanded 
films at higher temperatures. A two dimensional vaporization. Areas taken 
at low compression (1.4 dynes per cm.). 


20° this corresponds to about 18, and at 28° to about 25 dynes per 
centimeter. Figure 138 represents the data from an experiment 
in which a film of palmitic acid was kept at low compression, 1.4 
dynes per centimeter, and heated from 0 to over 50°. It will 
be seen that an increase of temperature produces no perceptible 
increase of area for the condensed film, a considerable increase 
in area in the expanded film, and a much more rapid increase 
during the transition. This is in accord with the relations found 
in the volume expansion of a liquid, of a gas, and during the 
transition from liquid to gas. 


FILMS AND THE SPREADING OF LIQUIDS ON SURFACES 


The spreading of a liquid as a film upon the surface of a solid 
or another liquid is a phenomenon which is of importance not 
only in nature, but also in connection with many technical 
processes. ‘Thus, it is difficult for oil to penetrate sand which is 


170 COLLOIDAL BEHAVIOR 


already impregnated with oil, and for water to penetrate sand 
wet by oil. Also a fundamental characteristic of a good lubricant 
is that it must spread over the solid surfaces to be lubricated and 
adhere well to them. 

Four different expressions of the criterion of spreading are to be 
found in the literature. Since these are not all in agreement, they 
cannot all be correct. | 

1. All liquids spread on a pure surface. 

2. A liquid b will spread on a liquid aif Ta>T> + Ta, where Ta 
represents the interfacial tension between the two liquids, and 
T, and 7; the respective surface tensions. The condition for 
non-spreading is T4<T> + Tos. 

3. Liquids whose molecules are polar, or contain polar groups, 
spread on water, while those without polar groups do not spread. 

4. A liquid will spread if its work of surface cohesion W¢ is less, 
and will not spread if its work of surface cohesion is greater, 
than its work of adhesion W, with respect to the surface of the 
liquid or solid upon which the spreading is to occur. The 
spreading coefficient, which under the conditions hereafter 
specified gives a measure of the tendency to spread, is defined as 


S =W.z-— We 


Criterion 4, developed by Harkins and Feldman," is justified 
both from the theoretical and from the experimental standpoint, 
and will be used as the basis of the discussion of this subject. 
Criterion 2, which is obtained by an application of the Neumann 
triangle of forces, corresponds numerically to criterion 4, but does 
not rest upon a sound theoretical basis. Criteria 1 and 3 are 
easily shown to be incorrect, although it seems that the latter of 
these two is of considerable importance in connection with the 
thickness of the film which is formed. Thus, the presence of a 
polar group in an organic molecule seems to be essential to 
spreading out to a monomolecular film upon water, but not for 
the formation of one which has a greater thickness. 

Criterion 4 as given above is easily developed upon the basis of 
thermodynamics. 

When a drop of liquid 6 is placed upon the surface of another 
liquid a, spreading may occur. If it does, the surface of the liquid 
a disappears, while its place is taken by substantially an equal 


iit cs rah 


SURFACE ENERGY IN COLLOID SYSTEMS lit 


area of the surface b plus an equal area of the interface ab, 
provided the surface of b and the interface ab do not lose their 
identity. If they do, then only one composite surface c takes the 
place of the surface a. 

The equations will first be developed for the case in which the 
film does not give a composite surface. 

The spreading coefficient may be developed by thermodynamic 
reasoning, provided the former of the two hypotheses of the pre- 
ceding paragraph is used as a basis. Since only large-scale 
motion is of importance in spreading, only the free surface energies 
are involved. The free energy decrease S which occurs in 
spreading is obviously given by the expression 


ya Yh. 1 Yah) (1) 


where yap represents the free energy of the surface or interface, 
since the right-hand side of this equation gives merely the net 
amount of free energy which disappears when the spreading 
occurs. The work of adhesion Wa, or the work necessary to pull 
apart the 1 sq. cm. of the interface ab, is given by the equation 
of Dupré as 

Ve = a 3. Yas (2) 


since all that occurs is the disappearance of the surfaces a and 6 
and the appearance of the interface ab. The work of cohesion is 
that necessary to create inside a liquid an area equal to 2 sq. cm., 
or, more specifically, to break apart a bar of liquid 1 sq. cm. in 
area in such a way as to give two surfaces, each 1 sq. cm. in area, 
and is given as 


co = 2 (3) 
A combination of (1), (2), and (8) gives 
S i Wa aad We 


which exhibits the extremely simple relation that spreading occurs 
if the adhesion between the two liquids is greater than the cohesion 
in the liquid which is in the position for spreading, while spreading 
does not occur if the cohesion is greater than the adhesion. It is 
obvious that a positive value of the spreading coefficient corre- 
sponds to spreading, a negative to non-spreading. It is also 
evident that because the liquid 6 spreads upon a, it is not at all a 


172 COLLOIDAL BEHAVIOR 


necessary conclusion that a spreads upon 6. Thus the spreading 
coefficient is given above for the case where a is the liquid the 
surface of which is already formed. The coefficient for a to 
spread upon b is S = y — (ya + Yas), SO a high surface energy 
for the liquid a acts in favor of spreading when a is the lower 
liquid, and against spreading when b is the lower liquid. Cor- 
responding with this, wt 1s found that almost all organic liquids 
spread upon water, while water spreads upon very few organic 
liquids. 

Definition of the Term ‘‘Film.”—A film exists whenever a 
layer, which has a different composition from the body of the 
liquid or solid, is present at the boundary surface, provided 
the area and form of this layer are independent of the gravitational 
forcesacting. Whenever the area and the form of the layer depend 
upon both the surface and the gravitational forces, a lens exists. 
If the area and the form of the layer are determined primarily 
by the containing vessel, the phase may be said to be present in 
bulk. 

Films may be said to be non-composite and composite. In a 
non-composite film the total surface energy is additive, in that 
it is equal to the value of the interfacial energy when both phases 
are present in bulk, plus the value of the surface energy which 
film-forming material possesses when it exists in bulk in equilib- 
rium with the phase upon which the film rests. In a composite 
film the total surface energy is less than this—so films may be 
very different in their degree of compositeness. 

Since the distinction between a film and a lens, as given in the 
last paragraph, may not seem to be sufficiently definite, it will 
be given below in a slightly different form. The layer of liquid 
at a phase boundary may be considered to constitute a film when- 
ever the gravitational forces which tend to change its form or area 
are inappreciable in comparison with the surface forces which are 
active. 

Of the 89 values of the spreading coefficient thus far deter- 
mined, shown in Table III, only two have a numerical value 
less than 1 erg, which corresponds to aforce of 1 dyne per 
centimeter. It will be seen that when the thickness of the upper 
layer is as great as ly, or 10-4 cm., or approximately 1,000 mole- 
cules, the layer may still be characterized as a film, since the gravi- 


SURFACE ENERGY IN COLLOID SYSTEMS 173 


tational force produced is of the order of only 4 & 107° dynes in 
the case of an organic liquid or water. Even when the thickness 
is 10u, or 10-%cm., the gravitational effect is only about 0.0005 
dynes percentimeter. Abundant evidence has been obtained both 
in this laboratory and by Langmuir, that the range of molecular 
forces for appreciable effects is very minute, and less than the 
distance across ordinary molecules. It is thus probable that such 
forces are inappreciable at distances of 10 X 10-§cm. Thus, a 
film may have a thickness more than a thousand times the 
range of molecular forces, and so may have complete independence 
with respect to the upper surface of the film and its interface 
with the lower liquid. 

From this point of view the spreading coefficient should be 
entirely significant at thicknesses between the upper limit of an 
order of 1,000 molecules or more, and some lower limit, which 
probably approaches closely to only a few molecules in 
thickness. 

The use of the spreading coefficient as an index of spreading 
would be justified if a large number of spreading coefficients could 
be determined with considerable accuracy, provided the liquids 
spread when the coefficient is positive, and do not spread when it 
is negative. Such a justification has been obtained, for it is 
shown that this is true of the 89 liquids listed in Table III; only 
one exception is found, and in that case the magnitude of the 
spreading coefficient is so low that its sign is doubtful. 

The coefficients of spreading listed in Table III apply only to tke 
spreading of the pure liquid upon an entirely clean water surface. 
If the surface of the water is impure, then the surface tension of 
the water, which occurs as a positive term in the spreading coeffi- 
cient equation, is lowered, so the coefficient which should be used 
in this case has a lower value than the one given. ‘Thus, it has 
in no case been found that a pure liquid with a negative coefficient 
will spread, but it is often found in rough experiments that a 
liquid with a positive coefficient will not spread, due to the presence 
of a slight impurity on the surface of the water. As organic 
liquids also, even when purified with great care, often differ 
slightly in their purity, the spreading coefficient relative to water 
should be determined by the use of a part of the same sample of 
liquid as was used in the experiment on spreading. 


174 COLLOIDAL BEHAVIOR 


Taste III].—Tue Spreapina CoErFrFIcient oF OrGanic LieuIDS ON 
WaTER* aT 20° 


A. Spreading Liquids 


S or S or 
WA-—We Wa-— We 
Ethyl aléohol...3. 00.202. )... — 60240 Ethyl capronate............ 25.64 
Methyl alcohols... 022... 50.10 Mercaptan.,\4 52 eee 24.86 
Propyl_ aliokobis one ce ¢ 49.10 Oleic acid. ......e ee ee 24.62 
Dipropylamine.............. 48.60 Iso-amyl butyrate........... 24.61 
Butyl alaoholss. 4. eee aso Aniline. ..... 30) sae eee 24.45 
Iso-butyl alcohol............ 48.20 Heptane......: i056 eos oa eee 
Propioni¢ acide. 2 oe ee 45.77 Ethyl! nonylates v7.5.3 see 20.88 
Butyric#eid fae. Gh wie ke 8 Trimethyl-ethylene.......... 18.85 
thy] ethers. bs cee ee SO Methylene chloride.......... 17.97 
ACSI Abid? sea ee 45.20 Ethyl bromide. .........)... 9% 44 
Acetonitrile. 52. nase 44.40 Benzaldehyde.,,.:;.....<...5 37.98 
Iso-amyl aleohol............ 44.30 Iso-amy] nitrate............ 14.82 
dso-valerie acid./... 1. oes 43.89 Chloroform. /.0.. a ee ee 
Methyl ‘ketone-<: i. i=. sera de a7 Anisole. . ... dense 176 
Di-isobutyl amine........... 40.47 Phenstole:., 44.2505 ae 10.66 
Methylbutyl ketone......... 37.58 p-Cymene. .3.5 30. ee 
Sym-octyl alcohol........... Shae Iso-pentane 2.45 eee 9.44 
Heptyli¢ acid $05. ¢a.. ve ani ie Benzol ... 2see ee ee 8.94 
Methylhexyl carbinol........ 36.67 o~Xylené. J. 2 Fede ee 6.85 
N-ottyl alecholn sist da. oe 35.74 Toluene: ois ae eee 6.84 
Formic acids tne 35.50 **Higher” paraffin........-..-. 6.72 
Butyronitrile).........0 0... 34.36 ‘p-XVlene ss es ee 6.70 
Iso-amyl chloride........... 33.88 Tetrachloro-ethane.......... 6.44 
Ethylpropyl ketone.......... 33.75 m-Xylene; a7 a ae ee ee 6.19 
Ethyl carbonate... ke 33.63 Ethyl benzol, mesitylene ..... 5.59 
Iso-valeronitrile.}..2.:<..... “82.63 Trichloro-ethylene........... 5.09 
Heptaldehyde..:.........5. o2c22 o-Nitrotoluene.............. 4.15 
Undecylenic acid (at 25°).... 32.02 m-Nitrotoluene.-/.- 2.40 4°13 
Methylhexyl ketone......... 31.92 Nitrobénzéne 225-1 are eee 8.16 
Ethyl iso-valerate........... 30.71 Di-iso-amyl (decane)........ 3.76 
Monochloro-acetone......... 30.42 Hexane, . 2 2) 1 eee 3.41 
Tert-butyl chloride........., 29.46 Chlorobenzene. ....7) 0,72) 2281 
Asym-dichloro-acetone....... 26.46 6, B’-dichloro-ethyl sulfide. ... 1.62 
Iso-butyl chloride........... 26.43 Pentachloro-ethane.......... 0.67 
Nitromethane is.44) open 26.32 Octane. ic ccc ae 0.22 
B. Liquids Which Form Lenses on Water 
Carbon tetrachloride........ 1.06(?) Tribromohydrin, 4, 22 —11.06 
p-Bromotoluene.....:....... — 1.29(80°)  ““'Stanclas 7 5 —13.44 
Ethylene dibromide......... — 3.19 Liquid petrolatum, Squibb’s.. —13.64 
Monobromobenzene......... — 3.29 a-Monobromo-naphthalene .. —13.86 
o-Monobromotoluene........ — 4.20 ; 
Acetylene tetrabromide...... —15.64 

Perchloro-ethylene.......... — 6.42 Methyl ‘odid 3 
Carbon disulfide............ =G6. 904 Cen eR SO eee #0258 
Phenyl mustard oil. ........,'— 7.68 Diphenyl methane.......... 
Monoiodobenzene........... — 8.74 Diphenyl dichloromethane. . . 
Bromoforni (3.55.0) oy ee ae Tribromo-ethylene......... . 
a-Monochloro-naphthalene... — 9.74 p-Bromotoluene at 30°....... = 1229 


* The values of Wc, from which the spreading coefficients were calculated, relate to the 
pure dry organic liquids, but the latter in spreading become saturated with water, there- 
fore still more exact information in regard to spreading would be given if Wo were deter- 
mined by the use of liquids saturated with water. 


al 


SURFACE ENERGY IN COLLOID SYSTEMS 175 


SPREADING AS RELATED TO THE PRESENCE OF POLAR GROUPS IN 
THE MOLECULE 


The 71 liquids listed in the first section of Table III were found 
by careful experiment to spread on the surface of pure water. 
These 71 liquids include hexane, octane, a higher paraffin, 
benzol, zso-pentane, toluene, p- and m-xylene, decane (di-cso- 
amyl), ethyl benzene, chlorobenzene, iso-butyl chloride, tertiary 
butyl chloride, zso-amyl chloride—a sufficient list to prove that the 
presence of a polar group ts not essential for spreading. 

One of the principal effects of the presence of a polar group is 
to increase the work of adhesion (W4). Since, when a very polar 
group, such as —OH, —COOH, —CONH, —CHO, —CN, 
—CON Hp, etc., is present, W. is very high, the term We in the 
equation S = W, — Wc is never large enough to give a negative 
value of the spreading coefficient. Nevertheless, when the work 
of adhesion toward water is small, the liquid may still spread if W- 
is still smaller. Thus, hexane, for which the value of W4 is very 
small (40.23 ergs), spreads, since W<¢ is extremely small (36.86), 
and the value of S is 3.387. The work of cohesion in octyl alcohol 
is nearly 20 ergs greater, so octyl alcohol is able to spread only 
because W, is also greater (by the remarkably great value 51.71) 
than that for hexane. ‘This illustrates the fact that the extremely 
great effect of the presence of a polar group in producing spread- 
ing is due to the fact that, in general, it increases the work of 
adhesion toward water very much more than it increases the work 
of cohesion. 

One of the most important factors in determining the magni- 
tude of the spreading coefficient toward water is the dissymmetry 
of the molecule. In general, the value of the coefficient increases 
as the electromagnetic field of force around the molecule becomes 
more unsymmetrical. This is due to the fact that with unsym- 
metrical molecules the work of adhesion toward water is much 
greater, in comparison with the work of cohesion, than in the 
case of symmetrical molecules, since, when the liquid is torn 
from water, the strongest field must be ruptured, while, when it 
is separated from itself, only the weakest field is broken. 


176 COLLOIDAL BEHAVIOR 


NON-SPREADING LIQUIDS 


It has been indicated in the preceding paragraph that one of the 
important factors in producing a non-spreading liquid is that the 
intensity of the electrical field around the molecule shall be 
distributed symmetrically in the case of the upper phase. 

It will be seen that, as with the paraffins, spreading may be due 
to a low value, less than 50, of the free energy of attraction toward 
water (work of adhesion), but in many more instances is brought 
about when this value is as high (about 75) as if they were esters, 
which spread to a monomolecular film. Thus -non-spreading is 
usually due to a high value of the work of cohesion of the substance. 

Non-spreading seems to accompany the presence of the =S or 
=CS or phenyl groups, or that of chlorine, bromine, or iodine, 
as substituents in paraffins, in benzol, or in naphthalene, even 
when the unsubstituted compound spreads easily. When only 
one chlorine atom is present in a paraffin derivative it seems to be 
polar and produces the opposite effect, considerably increasing 
the spreading coefficient, while with several chlorine atoms the 
coefficient is decreased. Bromine, and especially iodine, are 
much more effective than chlorine as substituents for producing 
non-spreading. ‘The above groups are evidently of the type 
which have a high attraction for themselves without having an 
especially high attraction for water. 


INSOLUBILITY AS AN ACCOMPANIMENT oF NON-SPREADING 


It would seem that the difference between the adhesional work 
and the cohesional work (W4 — Wc) should be important as a 
factor in determining solubility (though not so important as in 
the case of spreading), since the solubility of a substance also 
seems to depend on the difference between the attraction of the 
solute for the solvent and for itself. However, there is this 
distinction: In spreading on water it is the most active or polar 
part of the molecule which is chiefly involved, while the whole 
molecule takes part in solution. From this standpoint it is to be 
expected that spreading on water is a more common phenomenon 
than a considerable solubility in water, since spreading is a 
solution of only the most active or soluble part of the molecule. 


SURFACE ENERGY IN COLLOID SYSTEMS Pil 


In spreading it is not necessary for the molecules of the solute 
to penetrate between and push apart those of the solvent, as 
must be done in solution. 

Corresponding with this, it is found that all non-spreading 
liquids are practically insoluble. Liquids with very high spread- 
ing coefficients with reference to water are miscible with it, pro- 
vided the slightly polar (homopolar) part of the molecule is 
sufficiently small. Although the value of the coefficient for ether 
is moderately high (45), it is not miscible with water, since two 
ethyl groups are present in the molecule. Most liquids whose 
coefficients have positive values less than 10 are insoluble or 
only slightly soluble in water. 


EFFECT OF IMPURITIES ON SPREADING 


That impurities on the surface prevent spreading has been 
pointed out by many investigators; that active impurities in a 
non-spreading liquid may cause it to spread has been shown 
by Hardy and others. A simple and beautiful experiment illus- 
trates the latter effect. A large lens of “Liquid Petrolatum, 
Squibb’’—presumably any other oil with a high negative coeffi- 
cient would give similar effects—is formed in the middle of the 
surface of a sheet of water ina large tray. A drop of oleic acid is 
then placed upon the center of this lens. After a short period, 
considerable movement is noticed adjacent to this point and then, 
very suddenly, the lens is broken up into a great number of 
fragments which seem to be projected with almost explosive 
violence toward the edges of the tray. If the oleic acid is mixed 
with the oil before it is put on the surface, the material separates 
into a large number of minute drops, separated by a thin film, 
the drops moving constantly on the surface. The thin film 
evidently contains a considerable proportion of oleic acid. 


Ture SPREADING OF LIQUIDS UPON THE SURFACE OF A METAL 


The spreading of liquids upon the surface of a metal is of 
particular interest in connection with the problem of lubrication 
and that of flotation. Experiments with mercury’ show that 
the spreading coefficient for water (32) is high, and much higher 


178 COLLOIDAL BEHAVIOR 


(from 60 to 137) for all of the 29 organic liquids tested. Thus, 
all of these liquids, and probably all other organic liquids, 
should spread upon this metal, and, presumably, upon the surface 
of other metals. Careful experiments with 23 of these liquids, 
including water, resulted in spreading in every case, as predicted 
by the positive value of the spreading coefficient. Water does 
not spread upon an ordinary surface on mercury on account of 
the contamination of the surface by various substances, but 
spreads readily when the mercury is distilled in a vacuum in clean 
vessels, as was found by Rayleigh.*®° It is of interest to note that 
the higher alcohols and acids, which spread so readily on water, 
have specially high spreading coefficients on mercury, while, on 
the other hand, the presence of bromine or iodine in the molecule, 
which results in non-spreading with water, gives the greatest 
tendency to spread upon mercury. 

The work necessary to separate an organic liquid from mercury 
is especially high for iodine, bromine, sulfur, and carboxyl deriva- 
tives, which indicates that these groups are oriented toward the 
surface of the metal. 


THE NON-SPREADING OF WATER ON ORGANIC LIQUIDS 


The spreading coefficient for the spreading of water on an 
organic liquid is negative in all known cases, which indicates 
that water will not spread upon the surface of any organic liquid - 
when the two are mutually insoluble. Corresponding with this, 
water was found to spread upon the surface of only one of 18 
organic liquids tested, and this one was acetone, which is miscible 
with water. When a small drop of water is placed upon the 
surface of any organic liquid which will not spread on water, the 
drop remains upon the surface in a nearly spherical form. I, 
however, the organic liquid spreads on water, it will be seen that 
after the water drop is placed upon its surface, the organic liquid 
spreads as a film over the surface of the drop, and this then sinks 
if it is heavier than the organic liquid, but floats as a practically 
spherical drop if it is lighter. 


Herat or ADSORPTION 


An important phenomenon in which surface energy relations 
are involved is the liberation of heat which accompanies adsorp- 


SURFACE ENERGY IN COLLOID SYSTEMS 179 


tion. This may be illustrated by citing what occurs when lumps 
of outgassed charcoal are dropped into a liquid, though in this 
case the heat liberated does not correspond to the formation of a 
monomolecular film on the surface of a plane solid, so it may be 
more properly designated as the heat of immersion. One gram 
of bone charcoal, which had not been outgassed, gave a heat of 
immersion of 18.5 cal. in water, while the data of Lamb and 
Coolidge indicate a value of about 35 cal. for the heat of immer- 
sion of outgassed coconut-shell charcoal in carbon disulfide. 
The heat of immersion of a solid, the surface of which is so 
nearly plane that its surface energy is essentially equal to that of 
the same area of a plane surface of the same material, or the heat 
of adsorption of a liquid on the surface of the solid, may be defined 
in a corresponding way as the amount of heat liberated (—Q,) 
when a solid with a surface of this type, and of an area of 1 sq.cm., 
is immersed in a liquid in such a way as not to increase mate- 
rially the area of the surface of the liquid. In this process the sur- 
face of the solid would disappear, and in its place would appear the 
same area of interface solid-liquid. The heat liberated (— Qa.) 
would be equal to the total amount of energy given off in the 
process when carried out isothermally (#.), and this is equal to 
the total surface energy of the solid (#,), minus the total surface 
energy of the interface (H;), for 1 sq. cm. of surface. Since the 
total surface energy is always equal to the free surface energy 
(y) plus the latent heat of the surface (— T oh = 1), the following 


equation expresses the value of the heat of adsorption. 
—Q, = EF, = E, — Hy = ys +l — (vs + Is) a 


Ye — TOM — y, + TO (1) 

It is obvious that this equation is also valid for the heat of 
immersion of a liquid, or for the heat of adsorption of one liquid 
on the surface of another liquid, so in its more general sense the 
subscript s refers to the phase whose surface is already developed, 
but later disappears, giving place to an interface of the same 
area. The heat liberated on the immersion of a solid has always 
been found to be a positive quantity, which indicates that the 
total interfacial energy per unit area is always less, so long as 








180 COLLOIDAL BEHAVIOR 


this holds true, than the total surface energy of the solid. That 
this is not always the correct sign of the effect, at least when only 
liquids are involved, is shown by the fact that hexane, octane, 
and carbon tetrachloride have negative heats of immersion in 
water equal, respectively, to —0.21, —0.21, —0.26 times 10~-° 
cal. per square centimeter at 20°, though the heats of immersion 
of water in these liquids are all positive, 1.4, 1.34, and 1.05 times 
10~° cal. per square centimeter. Even in the case of two liquids, 
heat is almost always evolved on immersion as the result of the 
surface energy changes. Thus, for example, the heat of immer- 
sion of normal octyl alcohol at 20° in water is 1.28, and of water in 
octyl alcohol, 2.85, in terms of the units used above. 

The heat liberated on adhesion (—Q,), and the total adhesional 
energy (#4), are always larger positive (or smaller negative) 
quantities than those which give the heat liberated on immersion 
(heat of adsorption), provided the surfaces are plane. The 
following equation gives the heat of adhesion. 


G4 = Hy, =H, + Hi — #E;=(y7+h) ++) — 
(y; +1) (2) 


These are the same as the heat and energy of approach, since the 
surfaces of two phases already in existence approach each other 
and disappear, while an interface, equal in area to that of either 
surface which disappears, takes their place. The heat of adhe- 
sion is 2.6 for water-hexane, 4.0 for water-octyl alcohol, 2.5 for 
water-carbon tetrachloride, all in 10~® cal. per square centimeter 
aL20). 

When a liquid spreads over a solid, the surface of the solid 
disappears, while an interface of the same area appears. If the 
solid has a plane surface, then a liquid surface of the same area 
also appears; so, provided the liquid layer is not too thin, the 
following equation holds 


—Q:p = Bey = E, — (Ei + FH) =y,. +1 — (Gi th) — 
(y; +L) = HK, — E, 


where FE, represents the energy of surface cohesion of the liquid. 
Obviously 


—Q,. = EH, = LE, — Ey 


SURFACE ENERGY IN COLLOID SYSTEMS 181 


or the heat liberated by the adsorption of a liquid equals the 
energy of adhesion minus the surface energy of the liquid. Also 


‘aN As = Ky = Hep + Ei 
and 
Ep = E, — 2H; 


The heat of adsorption of a saturated vapor is 
=o, ne (y, + 1.) aoe (yi + 1) << AD = H, — H; + dv 


where —Q, is the heat of adsorption of enough vapor to form a 
liquid in bulk covering the solid surface at constant tempera- 
ture, and ) is the latent heat absorbed in the vaporization of the 
liquid per unit volume of vapor. It is assumed here that the 
area of the surface of the liquid formed is negligible in comparison 
with the area of the interface which is formed. 

The heat of adsorption as defined above is thus found to be 3 25 
for zso-butyl alcohol, 2.60 for secondary octyl] alcohol, and 3.13 for 
octane, all in 10-° cal. per square centimeter. The heat of 
spreading of n-octyl alcohol on water is about that of 7so-butyl 
alcohol on mercury, but that for octane on water is less than half 
the similar value for mercury. 


ADSORPTION 


The principle of minimal potential finds one of its most fruitful 
applications in connection with the distribution of a substance 
between a phase and its surface. The fundamental relations 
involved were deduced by Gibbs,‘ and are best known in connec- 
tion with his equation for adsorption. It has been found by 
Traube!® and other investigators that many substances, for 
example, the organic acids, alcohols, and amines, greatly lower 
the surface tension, and, therefore, the free surface energy of 
water, and that this lowering increases as the concentration of 
the solution increases. If a solution of such a surface active 
substance could be prepared at first in such a way that the 
concentration is the same at the surface as in the interior, then 
the free energy would be decreased by a movement of solute into 
the surface, that is, by a decrease in the concentration of the 
interior and an increase in the concentration at the surface. 


182 COLLOIDAL BEHAVIOR 


The change would proceed until the free energy of the solute in 
the interior is lowered and that on the surface is increased to a 
like value in both. An increase in the concentration of the solu- 
tion increases the osmotic pressure and the free energy, or it 
may be said that it increases the escaping tendency of the 
solute. This brings about a corresponding increase in the escap- 
ing tendency of the substances in the film. Over a considerable 
range of concentration in the solution, the concentration in the 
film is apparently constant, but this is only in the range in which 
the film consists of a monomolecular layer of the adsorbed sub- 
stance. Here the concentration of the water is so low that it 
may vary greatly, even though the variation is inappreciable in 
terms of the solute. Also the escaping tendency of the adsorbed 
substance may be affected by a change in its packing along the 
surface without any marked difference in surface density. This 
is analogous to the fact that, while a marked increase in pressure 
produces a considerable density change in a gas, the change in the 
density of a liquid may be inappreciable if only rough methods are 
used for the density determinations. 

In dealing with adsorption, both phases and phase boundaries 
are involved, and these may be considered as regions.'9 Thus, 
when we deal with a beaker of water, six regions are involved 
when the support for the beaker is neglected. These are the 
regions (1) glass, (2) water, (8) air and water vapor, (4) the 
interface glass-water, (5) the interface glass-air vapor, and (6) 
the interface water-air vapor. ‘Thus, for the three phases, there 
are Six regions. 

The equation of Gibbs may be developed by a consideration of 
the dilution or concentration of the solution by means of a piston 
provided with a semi-permeable membrane, and of the variation 
in the area of the surface by means of a variable float or barrier. 
The thermodynamic relations between the energy changes involved 
give the desired equations when suitably combined. The most 
complete development of this kind, since it is the only one which 
deals with the phases on both sides of the interface, was produced 
by A. C. Lunn? at the request of the writer. It is given below. 
Itis based on the laws of thermodynamics and the equation 
for maximum work considered in connection with osmotic 
pressure. , 


idea a3 


SURFACE ENERGY IN COLLOID SYSTEMS 183 


Notation: \ = latent heat 


p. = adsorption of film 
a = area 
y = surface tension 
9,p’ = osmotic pressures | 
Ll’ = latent heats 
v,v’ = volumes | of volume phases 


ry = dilutions, or reciprocals of 
concentrations (c,c’) 

s = entropy 

c = thermal capacity of system 
M = total mass of solute 

q = heat added to system 

w = work done by system 

Y = Helmholtz free energy 





The first law of thermodynamics may be stated in the form 
dg=cdt+ldr+l' dr + dda Cl) 

The equation for reversible work is 
dw =p dv+ yp’ dv’ — yda (2) 


The negative of the differential of the function of Gibbs, or of 
the Helmholtz free energy, is defined as 


— dy = d(st) — (dq — dw) 
= sdt+tds — (dq — ydw) 
= s di + dw (3) 
=sdt+pdv-+p’' dv’ — yda (3b) 


The total mass of solute (/) is given by the following equation: 


M =vc+0'c’ + wa 
or 


eto Ad, = ita (4) 
Since r and r’ are related at a given temperature, and also p 


and p’, the independent variables may be taken as ft, r, v, and a. 
Then p, p’, l,l’, \, u, and y will be functions of r and ¢. 


184 COLLOIDAL BEHAVIOR 


The mass of substance in the phase represented by primed 
letters is given as follows: 


m’ = v'¢! = 5=M-4ya—* 
SO 
y = (u — pa -*) (5) 
so 
dy’ = (2 i + a)(M — ya —°) 
a ane — °; ar _ raha 
ds — r’uda (6) 
2, (a1 an 2) — eb 
[ar (mf — ma — 2) —ri( ast 5) Jar 
— ao — 7’ uda (7) 
- Substituting (7) in (3b): 
= —dy =|{s +p [2 (mM = a —")— rast 
sole Hee 
+(p se PTY ay — (y + p’r'p)da (8) 


In order that this may be an exact differential, the following 
conditions must be met—(9), (10), (11), (12): 


Sot of (arm ~1)- vet] 
“abn 9 eC - aM) © 
slr +713 (" —m ~5) ro = Sle 22) 


o fst p'[ (um — ya —*) es any = — Sy + pln) (11) 


Aone 


SURFACE ENERGY IN COLLOID SYSTEMS 185 


Beet < oo —*)- (0% -2)]] - 200-22) on 
gat PL gr (Me — wa —2)— (age 5) ]} = —5.(7 + v'rn) cas) 
+(0 = ze) Sr Sy a" OO) (14) 


(14) is identically satisfied as 0 = 0. 
(10) and (11) give 




















p' or’ D Ora reo. 
or Oi» 8p drs Orr’ | p’r’ 
Ear Sp et we: 
emo OP Oy), Oe | Op’, ts, or’ 
| er aor! ar or PH, (16) 
or 
rap’ 
Op or 
Or =e : — 0 (15") 
0 Op’ 
pie = 0 (16’) 
ape) TO 
On” fo a 
and 
Ws oy 
or or 
rea anlar ~ rap (18) 
or or 


or the adsorption in mols is equal to the concentration of the 
solution times the rate of increase of the surface tension with the 
dilution divided by the rate of decrease of the osmotic pressure 
with the dilution. 

(17) gives 


y! u) op ei 
~ = apr aa (19) 
Equation (19) may be written in the form 
acl EE Dae ara 
Cer (sp) sia 


or the rate of change of the osmotic pressure in one phase with 
respect to that in the other, at constant temperature, is equal 
to the ratio of the respective concentrations. 


186 COLLOIDAL BEHAVIOR 


In the special case where the van’t Hoff formula holds: 
pr = RT p’r' = RT 


ae ay eas i 3 
Spe eri ‘dn’ (20) 
p’ / 
isothermally, and © 5 is a function of ¢; oe is a function of ¢, 


Tee: 
which, changing r to ¢ Bives 


1 cdy 1 cdy 1 ere 1 





(21) 


Ye “RT de. RY de RE dine eee tas 


es ae Ye ter 


Wi MON 
WEA ANK 





Surface Tension 


|. Formic Acid 
cetic  » 
3. fe joric » 
UTYrIC 
ns A » 
6. Caproic » 
Te Heptyic yn» 
]O+——— 6. Nonyhe » 
9 Decylic » 





Log of Concentration 


Fia. 14.—Adsorption curves for fatty acids. 


The validity of this equation was tested by Donnan and 
Barker,?! by bubbling air through a solution of nonylic acid. 
They found by this direct method that the adsorption was 1.0 
xX 10-7 g. per square centimeter, while the Gibbs’ equation gave 
0.6 X 10-° ifthe value 2 was assumed for the factorz. The con- 


SURFACE ENERGY IN COLLOID SYSTEMS 187 


firmation was thus within the limits of error of the experiments 
and the assumptions. 

The surface tensions of solutions of organic acids, alcohols, 
and other similar substances have been investigated by Traube,!® 
Drucker,?? Whatmough,”° Szyszkowski,?4 and others. The mea- 
surements on the organic acids have been repeated and extended 
to decylic acid by Harkins, King, and Clark, and their results are 
given graphically in Fig. 14, which will be used as a basis for the 
discussion which follows. Curves 5 and 6 are plotted from the 


Weight, groms 





Fia. 15.—Effect of time on the drop weight of decylic acid, 0.00015 n. 


data of Drucker, and it will be noted that the curve for capryllic 
acid, with 8 carbon atoms, is missing. ‘The data were determined 
by the drop weight method, since the capillary height method 
proved inaccurate for the higher acids. Figure 15, which was. 
obtained in connection with the experiments, illustrates the 
fact that adsorption is a process which occupies considerable time, 
the time to obtain approximate equilibrium increasing with 
great rapidity with the length of the hydrocarbon chain. Thus, 
equilibrium was not established with a 0.0015 wn solution of 
decylic acid in the course of half an hour. This makes the deter- 
minations tedious, since the drop must be held suspended in 
saturated vapor for a very long time. The slowness in obtaining 
equilibrium is due to the fact that a highly concentrated film must 
be formed, consisting of practically a film of acid, by diffusion 


188 COLLOIDAL BEHAVIOR 


from a solution which contains only 1 molecule of acid in 360,000 
molecules of water. 

From Milner’s result?’ for acetic acid, Langmuir" calculates 
that there are 2.3 X 10!* molecules per square centimeter, or the 
area occupied per molecule is 43 A.? U., while from Szyszkowski’s 
equation he gets the area 31 A.’ U. for the acids with 3, 4, 5, and 6 
carbon atoms. ‘These areas are not very different from that 
(21.6 A.? U.) which he obtained for the higher fatty acids, so he 
reaches the conclusion that the surface is covered with a mono- 
molecular film of the acid over the region in which the slope of the 
curves (as in Fig. 12) remains constant over a considerable range 
of concentration. He considers that the rate at which molecules 
escape from the liquid phase into the surface is proportional to 
the concentration of the solution, but the rate at which they 
return from the surface back to the interior depends upon the 
number of molecules in the surface, but is also dependent to a 
very great degree upon the difference in the potential energy of 
the molecule in the two states. 

From Traube’s data, Langmuir calculated the areas occupied 
by a molecule for 24 organic substances. The results for a part 
of these are listed below. 





| Area in A.? U. 




















No. C atoms — 
Normal acid Iso-acid Alcohol | Iso-alcohol 
see 
2 cae 32.0 
3 33.8 ns 29.1 34.7 
4 31.2 31,2 27.8 
5 ihte 30.5 27.8 





For comparison, the results obtained for the normal acids and 
alcohols by Harkins, King, and Clark are also given. The results 
for formic and acetic acid are only approximate, since, for the 
concentrations used, the osmotic pressure is not proportional 
to the concentration of the solutions. For acids above 10 carbon 
atoms, the results were obtained from surface films by Adam. 


SURFACE ENERGY IN COLLOID SYSTEMS 189 














Area in A.? U. 
No. C atoms 
Acids Alcohols 

1 Dieu | 
2 50.0 
3 39.0 | 
4 36.0 28 
5 (3270) 
6 (31.0) 
7 34.0 
8 meD 34 
9 oo 

10 rosd Gn 

14 201 

LS Pde tan k 

18 Awa 

Ze. paged N 


(Values in parentheses from data by Drucker.) 


These results are just the opposite of what would be expected if 
orientation were absent, since the area per molecule decreases 
with increase in the size of the molecule. This indicates, as has 
been pointed out by Langmuir and Adam, that the long chains 
hold together better than the short ones. 

By taking into account the kinetic equilibrium between the 
surface layer and the interior of the solution, and by the use of 
an empirical equation of Szyszkowski, Langmuir calculated the 
decrease in potential energy which occurs when a gram molecule 
of solute passes from the interior into the surface film. This 
decrease in potential energy becomes greater and greater as the 
curves in Fig. 12 shift toward the region of lower concentrations. 
It will be seen that the shift in the logarithm of the concentra- 
tion is equal to 0.555 per CH» group added. Langmuir calcu- 
lates that this corresponds to 625 cal., and that 


Nee Ag en it 


where } is the decrease in potential energy, and X, is the value 
given below: 


190 COLLOIDAL BEHAVIOR 


VALUES OF \, IN CALORIES PER Mou 


Tertiary alcohol... v.. . 04. nego see 950 
Primary. amine 33s)... eee ee ee ey © nee aa 600 
Primary alcohol). ~:.: (ass. ee oe 575 
Hster. 0 el ey 2 ee eee 470 
Monobasie: acid... a edidees 444s nie Oe 'Ss ae 437 
Ketones, 5s Ss wos bow gees a ae a 295 
Aldehyde... Fs hs ee ee 210 
Amide. 250 eo. oe Ee ae ae —510 
Dibasic acid or aleohol.< .. .).7%,.3. 0... —700 


The considerations given above give remarkably strong evi- 
dence that the molecules in such adsorbed films are oriented. 

Equally striking evidence has been obtained in an investigation 
by Harkins and King,** in which different types of interfaces are 
compared. It was found that the film of constant composition, 
or the monomolecular film, contains 2.78 <X 10'4 molecules per 


square centimeter at the interface between water and vapor, - 


and that between water and benzol there are 2.77 X 10!4 mole- 
cules. This is extremely remarkable, since it shows that the 
number of molecules in the film is independent of the nature of 
the second phase, which would be very difficult to understand 
upon any other basis than that the number is conditioned by 
the same closely packed and orderly arrangement of the mole- 
cules in both cases. 

While the number of molecules in the constant film is the same 
at the interfaces water-air, and water-benzol, the drop in potential 
experienced by a molecule when it passes from the water into 
the surface water-vapor is very much greater than when it goes 
into the interface water-benzol, that is, the surface of water 
is a very much more efficient trap for molecules of the polar- 
homopolar type, designated by the symbol , than the inter- 
face between water and benzol. 

This result may be considered from the standpoint of the 
intermolecular electromagnetic fields involved. Since cohesion 
depends upon the intensity of these fields, it may be considered 
that the fields are extremely weak in liquid helium, the liquid 
of lowest cohesion, and extremely high (interatomic fields) in 
diamond, in which the cohesion is the highest known. 

If two phases, A with the higher and B with the lower intensity 
of stray field uniting the molecules, could be put in contact in 


SURFACE ENERGY IN COLLOID SYSTEMS 191 


such a way as to have the field of each perfectly uniform up to a 
plane phase boundary or interface between them, then the whole 
drop in intensity between the two phases would occur in a surface 
of infinitesimal thickness; so, at least by certain methods of 
mathematical analysis, the free surface energy would be infinite. 
While this is not an actual case, it suggests the idea that, as the 
thickness of the transition layer increases, the free surface energy 
diminishes. With a given thickness of the surface layer, the free 
surface energy increases as the intensity of the stray field in B 
decreases, so the maximum free surface energy is reached when B 
is a vacuum, and is nearly realized when it is a dilute vapor or gas. 

In so far as the cohesion of a liquid is an index of the average 
intensity of the stray field in a liquid, it might be expected that 
the free surface energy between the given liquid A and a fluid 
phase B would thus increase as the cohesion in B decreases, provided 
the thickness of the surface film remains constant. The values of 
the cohesion which should be used in such a comparison are not, 
however, those for pure liquids, but should be the results obtained 
for the saturated solution of each liquid in the other—if equilib- 
rium values are desired. It is manifestly true that the equilib- 
rium value of the interfacial free energy between miscible 
liquids is always zero. 

With dilute solutions, however, this last condition is not impor- 
tant. Let the two phases be a and b, with the phase a consisting 
of the constituent A, which we will suppose to be water in all 
cases. Let the phase b be water vapor B in one case, and benzol 
B’inanother. 'Thedrop in intensity between the electromagnetic 
fields which meet at the interface between water and vapor is 
much greater than that between water and benzol, since the 
intensity in a dilute vapor is almost zero. ‘This is illustrated by 
Fig. 16. 

Suppose that a constituent C, a polar-homopolar substance 
such as butyric acid, is distributed between the two phases. 
If this passes into the interface in either case it makes the 
transition from one phase to the other less abrupt by increasing 
the thickness of the film. 

The drop in the intensity of the stray field shown in Fig. 16 may 
be considered as a restraining and an ortenting force, similar to that 
existing in the field in air between the north and south poles of a 


192 COLLOIDAL BEHAVIOR 


magnet; and the molecules which are orvented as similar to small 
magnetized needles between them. ‘The restraining force on the 
needles increases as the rate of fall of the magnetic intensity 
increases (increase of magnetic flux, or in the number of lines of 
force), so the restraining force holding the molecules might be 
expected to increase with the total drop of intensity of the stray 
field between the two phases, since the molecules of C in the 
interface bridge the total distance between the two phases. 








WATER PHASE 


BENZENE 
PHASE 


;Vapor Phase of Water 


Fic. 16.—The drop in intensity between the electromagnetic fields which meet at 
the interface between water and vapor, and between water and benzol. 


If the molecules of C are thus held in the interface by a restrain- 
ing force the molar activity or the molar fugacity (molar-escaping 
tendency) of C will be much less in the interface than in either 
of the two-volume phases, so that the concentration of C in the 
interface must be much greater than that in either of the volume 
phases to give a condition of equilibrium. 

Since the fall of intensity at the interface water-vapor ts much 
greater than that between water and benzol, the restraining force 
zs much greater in the former case, so the average activity of the 
molecules of C will be much less at the water-vapor interface; 
and, therefore, to give equilibrium, the diffusion pressure of C 
in the two phases water-vapor will be much less, to give a certain 
concentration of C in the interfacial film, than when the two 
phases are water-benzol. ‘This is true so long as the thickness of 
C does not increase beyond that of a monomolecular layer. 

The data obtained in the experiment indicate that in the 
monomolecular film between water and benzol the butyric acid 


SURFACE ENERGY IN COLLOID SYSTEMS 193 


molecules have about 3.5 times the activity which they have in 
the monomolecular film between water and vapor. This demon- 
strates that the water-vapor interface is by far the more efficient 
trap, even though the total number of molecules it can hold is 
conditioned by the area available, and is, therefore, no larger 
than the number at the interface water-benzol. 

A series of experiments by Harkins and Grafton has given the 
areas per molecule for the mono and dihydroxy derivatives, and 
one of the trihydroxy derivatives of benzol. The areas are 
as follows for the most concentrated films: 


ComMPouND AREA IN A. U. 
eet DIM ek Os asslade he sb ohne Be 36.6 
Voy tata NG) Cs a 55.3 
NGOS) 96.0 
a STS LCT RY A US Se (185.0) 
ee i, ek Sy eo ne Ws kee ene ds oe 42.7 


The result for hydroquinol is not comparable with the others, 
since the film for this substance has not reached the constant 
composition needed for a monomolecular film. It is evident that 
the area occupied by the dihydroxy compounds increases rapidly 
from ortho to meta to para, since the curve for hydroquinol 
indicates a much higher value than 96 in the constant Hae 
which is not reached on the saturated solution. 

The orientation of derivatives of benzol has been considered 
by Langmuir,'! and by Harkins, Davies, and Clark,!? but, on 
account of the fact that the structure of benzol is unknown, space 
cannot be given to such a complicated subject in this short 
review. It may be mentioned, however, that the surface energy 
of the dinitro benzols is highest for the meta, and lowest for the 
para, derivative, which seems to indicate that the active portions 
of the molecule are most easily buried when the two like groups 
are in the para position. With two different active groups which 
differ in activity, the para compound usually has the highest, 
instead of the lowest, total surface energy, which indicates, 
presumably, that the lack of balance between the groups in the 
para position causes one of them to be raised into the upver part 
of the surface. 


194 COLLOIDAL BEHAVIOR 
SurFACE ENERGY AND HypROGEN ION CONCENTRATION 


The importance of the hydrogen ion concentration in connec- 
tion with the phenomena exhibited by colloidal solutions has 
recently been emphasized by Loeb, and it is evident that acids 
and bases also play an important réle in influencing the surface 
and interfacial energy relations of certain types of solutions. 
For example, it was found by Harkins and Ewing’ that the 
interfacial tension between £8 dichloroethylsulfide (mustard 
gas) and water is lowered 55 per cent (from 28.36 to 12.78 dynes 
per centimeter) at 25°, if the aqueous phase is made 0.1 N with 
respect to sodium hydroxide, while a lowering of 35 per cent is 
produced if it is given the same concentration with respect to 
sodium carbonate. On the other side of the neutral point the 
interfacial tension is very slightly greater than for pure water, 
the increase due to tenthnormal hydrochloric acid being only 0.54 
dynes per centimeter. The use of a dilute alkaline solution of 
turkey red oil reduced the surface tension by 80 per cent, when 
the concentration of the oil in the water was 1 per cent, and of the 
sodium carbonate, 0.1 N. 

The interfacial tension between water and ethyl oleate 
was found by Harkins and Mulliken to be 21.34. With 
0.1 nN sodium hydroxide this value fell to the remarkably low 
value 0.3 dyne, possibly the lowest value ever obtained for an 
interfacial tension with an aqueous phase except when a soap is 
added. A plot of the interfacial tension against the logarithm 
of the OH- concentration has the form of a typical adsorption 
curve. The results with ethyl oleate are evidently not due to 
adsorption of OH™~ ions, but to the adsorption of sodium oleate 
formed by hydrolysis of the ester. Similar results, but not 
so marked, were given by ethyl capronate, and by methyl hexyl 
ketone, and to some extent by chloroform. With secondary 
octyl alcohol, neither acid nor alkali produced any appreciable 
change. 

The effect of the addition of acids or bases is very marked with 
certain solutions. For example, the surface tension of a 0.1 N 
solution of sodium nonylate is 20.18 dynes per centimeter. The 
addition of sodium hydroxide to the solution increases the sur- 
face tension with extreme rapidity, at 0.001 N to 33.37, and at 


SURFACE ENERGY IN COLLOID SYSTEMS 195 


0.008 N to 48.82. This effect is evidently due to the repression of 
the hydrolysis. Further increase in the concentration of the 
sodium hydroxide gradually reduces the surface tension, and in a 
linear relation to the hydroxy] ion concentration. 

The above results seem to indicate that the marked effects 
of acids and bases upon the interfacial or surface tensions of 
certain substances are due to chemical changes in the sub- 
stances much more than they are to the adsorption of hydrogen 
or hydroxyl ions. 

Table IV shows the remarkable effects of certain salts upon a 
film of sodium oleate. The data were obtained by Harkins and 
Thomas, and were secured by a repetition of the work of Clowes*! 
in such a way as to obtain surface energy values instead of the 
number of drops given by a certain pipette. 


TaBLEe I[V.—EFFECTS OF THE ADDITION OF SALTS UPON THE INTERFACIAL 
TENSION WITH A Fium or Sopium OLEATE (OLEIC AciIp Was 
AppDED TO Eacu Oi To A CONCENTRATION OF 0.001 mM) 





Surface tension in dynes per 


Base or salt added to aqueous phase : 
square centimeter 








NaOH NaCl CaCl, 
Olive oil Paraffin 
oil 

Mols per liter 











Ae yes i Be ea ee Eee 24.11 31.05 
CORO its OR es See CS: fice’? 

0.001 Aad Beye It A) Seems 0.002 0.00 
0.001 0.0015 9.88 9.65 
0.001 0.15 0.0015 6.88 7.48 
0.001 0.30 0.0030 6.36 7.12 
0.001 0.45 0.0045 6.70 7.36 
0.001 0.60 0.0060 Crow 8.20 


Thus while calcium chloride, the salt of a bivalent metal, 
increases the interfacial tension, sodium chloride reduces it very 
greatly. Thus when the aqueous phase was 0.001 molar with 
respect to sodium hydroxide, and 0.15 molar with respect to 


196 COLLOIDAL BEHAVIOR 


sodium chloride, the interfacial tension toward the oils 0.001 
molar with respect to oleic acid was 0.002 for olive oil, and 0.00, 
or too low to be measured, for the paraffin oil (purified stanolax). 


SURFACE ENERGY IN CoLLoIpD SYSTEMS 


As has been emphasized by numerous writers, the importance 
of the surface energy of any system increases rapidly asits particles 
become smaller. Thus, if a sphere of water 1 cm. in diameter is 
converted into a fog of minute drops 0.1u in diameter, the sur- 
face is increased 100,000 times, or to an area about 1 by 30 m. 
Accompanying this change the total surface energy increases 
from about 0.00009 cal., an amount altogether imperceptible in 
ordinary measurements of heat, to about 9 cal., which is nearly 
one-fourth the latent heat of fusion for this amount of liquid. 
For particles 0.01u in diameter the surface energy becomes more 
than twice the latent heat of fusion. 

There is no sharp distinction between systems which are and 
are not colloidal, but a colloidal system may be defined as one in 
which the surface energy is appreciable in comparison with the 
heat of fusion or of vaporization of the material present as the 
dispersed phase. 

According to the equation developed by W. Thomson, 

RT 1 Bre aie fy -) 

M Pi Pp \¥Fe i 
the vapor pressure of a particle depends upon its surface tension 
and increases as the size of the particle decreases. Thus, a drop 
of water of radius lu has a vapor pressure only 0.1 percent 
greater than a plane surface; for a radius 0.01, it is 10 per cent, 
and at luu, 100 per cent greater. 

The similar equation of Ostwald and Freundlich replaces the 
pressure p in this equation by the solubility s, and is 

ieee “l(a in =) 

M Sy p \f2 Ty 
Here M is the molecular weight of the substance, p its density, 
and r is the radius of the particle. Thus, it is evident that the 
relations in any colloidal system are highly dependent upon the 
magnitude of the surface tension. ? 











SURFACE ENERGY IN COLLOID SYSTEMS 197 


It is evident that the surface tension of a surface depends upon 
its curvature. Langmuir" considers that the shape of the mole- 
cules in the surface film determines the size of colloidal particles. 
Thus, in discussing the orientation theory he states: 


This theory also affords an explanation of the mechanism by which 
colloids are formed. If a film of closely packed oleic acid molecules 
covers the surface of water to which sodium hydroxide has been added, 
OH groups are absorbed by the COOH radicals, causing an expansion 
of the lower side of the film without a corresponding expansion of the 
upper side. This results in the bulging of the film downwards in spots, 
so that it finally detaches itself in the form of particles, the outer surface 
of which consists of COOH groups together with the absorbed OH, 
while the interior consists of the long hydrocarbon chains. 

The size of the colloidal particles is determined by the difference 
in size between the two ends of the molecules, just as the size of an 
arch is dependent upon the relative sizes of the two ends of the stones 
of which the arch is constructed. 


EMULSIONS 


Harkins, Davies, and Clark’? determined the surface energy 
variation with the concentration for sodium oleate, a substance 
of importance in the formation of emulsions, and came to the con- 
clusion that the formation of emulsions by emulsifying agents of 
the type of soaps is due to the formation around the drops of a 
film of adsorbed molecules (together with some molecules of the 
acid formed by hydrolysis, and some of the acid ions, oleate, 
stearate, and similar ions). The number of these ions is prob- 
ably not very great, since otherwise the oil drop would be more 
highly charged with negative electricity than is actually the case. 
Work upon the effects of acids and bases upon surface tension 
seemed to indicate that the great effect of hydrogen ion con- 
centration is not produced directly through the adsorption of 
hydrogen or hydroxyl ions, but through their effect upon the 
hydrolysis of the soap, which changes the surface tension mark- 
edly, as was shown in the section on the effects of acids and 
bases upon surface tension. The following quotation is of inter- 
est in giving some of the details of this theory. 


_In order to illustrate this effect, we have chosen for investigation a 
substance, sodium oleate, in which the paraffin chain is so long that it 


198 COLLOIDAL BEHAVIOR 


is very highly adsorbed. While many investigations of this kind have 
been made, in none of them, so far as we have been able to discover, 
have accurate methods been employed, and this substance was chosen 
because it is adsorbed so greatly that it is extremely efficient in the 
formation of emulsions. The hydrocarbon chain is so long that even 
though it is an unsaturated substance, oleic acid is insoluble in water, 
so that, since sodium oleate is partly, though not highly, hydrolyzed 
by water, there is the additional complication in this case that the acid 
may separate out as a colloid. It would be expected that the sodium 
oleate or the oleic acid, in so far as they are adsorbed in the molecular 
form, would be set with the —COOH or —COONa group toward the 
water. Now, the paraffin chain is so insoluble that, even when a colloid 
is not visible, it is found by investigations on the conductivity of such 
solutions (McBain) that the ions have formed very heavy multiply 
charged aggregates or micelles. These aggregates carry a negative 
charge, and would seem to consist of a considerable number of the nega- 
tive ions of the salt. The formation of these aggregates is quite likely 
connected with the insolubility of the hydrocarbon chains, and they form 
with the saturated compounds, such as sodium palmitate, just as they 
do with the oleate. 


This idea was also applied to the inversion of emulsions, and 
to their stability, as conditioned by the shape and orientation of 
the molecules. 


When Newman, working with Bancroft in 1914, found that, while 
sodium oleate in solution will give emulsions of benzene in water, and the 
oleate salts of a metal with a valence higher than one will give emulsions 
of water in benzol, we were working experimentally in this laboratory 
on the adsorption of these long hydrocarbon chains. Now, while Ban- 
croft seemed to think that this work indicated that the liquid with 
the higher surface tension forms the inner phase, it seemed to us that the 
only apparent relation was that to the number of oleate radicals in the 
molecule of the protective colloid (sodium oleate, or magnesium oleate). 
Therefore, it quite possibly may be the orientation—(according to the 
theory presented in this paper, molecular orientation in the interface 
is the factor which does determine the stability of emulsoid particles and 
also the sign and magnitude of the electromagnetic field at thesurface 
of the drop; it should be noticed that all of the best commercial emulsi- 
fying agents have very long molecules)—and the form of the molecules, 
together with adsorbed ions in the interface between the dispersoid parti- 
cles (or small drops), and the dispersion medium which determine the 
surface energy relations, and, therefore, the size of the drop at which it 
becomes stable. In other words, this idea is that the drop would be 


SURFACE ENERGY IN COLLOID SYSTEMS 199 


stable whenever the molecules, together with adsorbed ions, etc., in the 
interface fit the curvature of the drop. The molecules in the curved 
surface would not need to be all of the same kind. If the molecules do 


Curve A Solution in CeHe 
: roy B » » Hod 


Ergs perem® 
> 
om 


a E Interface C6 He -H>0 Solution 
10 , u - af, yD » 





Te Se rms 
0 0.01 0.02 0.03 0.04 0.05 00Gb 0.0F 0.038 0.09 010 


Equivalent Concentration 


Fig. 17.—The effect of sodium oleate upon the free surface energy of water, 
and also at the interface between water and benzol. The heavy horizontal 
lines indicate saturated films. (A saturated film is by definition one whose 
surface tension does not change with the concentration of the solution. All 
films that are not monomolecular are saturated.) Curve A indicates that a 
saturated solution of sodium oleate in benzol has practically the same surface 
tension as pure benzol. Curve D represents the interfacial tension between 
two layers obtained by rotating aqueous sodium oleate solutions with benzol 
and allowing to stand until the next day. Curve B shows the values for the 
surface tensions of the aqueous sodium oleate phase after rotating with the 
benzol as for Curve D. Curve FE indicates the interfacial tension between 
benzol and aqueous solutions of sodium oleate when no emulsification or rota- 
tion had taken place. Curve F represents the data for the interfacial tension 
between the layers after the aqueous solutions of sodium oleate were shaken 
vigorously with about one part of the benzol to 20 parts of the solution. This is 
the best of the methods studied for determining the interfacial tensions, since the 
benzol dissolves practically no sodium oleate. Curve C gives the data for the 
aqueous solutions of sodium oleate in air. It shows that the surface tension 
drops off very rapidly until at a concentration of about 0.002 N we have the 
lowest surface tension. 


not fit in the curved surface, the drop will not be perfectly stable, and 
will either decrease or increase in size if given time. 


200 COLLOIDAL BEHAVIOR 


From the standpoint of this idea of molecular orientation and molecu- 
lar fitting, the free surface energy of small drops should vary with the 
radius of curvature (in addition to the pressure effect which is usually 
taken into consideration), and we have been working on the surface 
energy relations of large drops (curvature so large as to be practically 
planes from the standpoint of the results) only as an introduction to 
work on the surface-tension relations at highly curved surfaces. 


Figure 17 presents some very interesting relations. Thus 
0.0001 m sodium oleate reduces the surface tension of water 
from 72.8 to 60.46 ergs per cm?., and even when 0.0002 m 
sodium hydroxide is added, to prevent hydrolysis, the surface 
tension goes as low as 61.32, so the oleate film builds up with 
extreme rapidity. A 0.014 m solution decreases the inter- 
facial tension from 35 to 2.22, or to about 6 per cent of its former 
value. Both the curve at the air-liquid and at the benzol 
aqueous solution interface, indicates that the adsorption is 
enormously rapid at first, and that for the vapor-solution inter- 
face at as low a concentration as 0.002 m the adsorbed film 
has become so closely packed that further increase in the 
concentration of the solution no longer lowers the surface ten- 
sion. ‘Thus these films become saturated at extremely low con- 
centrations of the saturating substance. These experiments 
show that sodium oleate will cause the benzene to form the 
emulsoid drops even when the outer phase has the higher surface 
tension when measured alone with a plane surface. It is also 
of interest that even for a plane surface the interfacial tension 
drops very low for the solutions which form stable emulsions, 
and that the value falls as low as 2 ergs. per cm?. or even less 
in one case, so the curvature of the surface would not have to 
produce a very large effect to reduce the surface tension to 
zero. 

Figure 6 represents this theory in the form of highly conven- 
tionalized drawings. The molecules act as oriented truncated 
cones, but in two dimensions as wedges, so this may be called 
the oriented wedge, or truncated cone, theory of emulsions. 
The conical shape of the molecule is conditioned by the relative 
areas of the cross-sections of the head, or polar part of the mole- 
cule, and of the hydrocarbon chains at the other end. The most 
pronounced changes occur when two or three CH;(CH2),COO— 


SURFACE ENERGY IN COLLOID SYSTEMS 201 


groups are substituted for one, since this always changes the emul- 
sion from drops of oil in water, to drops of water in oil. The size 
of the drop is not that which would be calculated from the shape 
of the molecular truncated cone, since the shape of the drop is 
affected by a staggering of the molecules in the film (subfigure 3), 
by the presence of some molecules of the acid, and by a few of the 
acid ions. It is found, for example, that drops of oil in water 
are much larger than would be expected from the shape of the 
molecular truncated cones. A further prominent factor in 
bringing this about is that molecules of oil from the drop undoubt- 
edly penetrate between the hydrocarbon chains of the soap and 
thus reduce the effect of the conical shape. This factor has not 
been taken account of in the drawing, which has been made as 
simple as possible. 

Finkel, Draper and Hildebrand have shown that the size of 
the drops, and the stability of the emulsions, vary with the 
dimensions of the polar head of the soap molecules in just the 
order specified by the theory proposed by Harkins. 

Figure 18 gives results which have an important bearing upon 
the application of the orientation theory to the subject of emul- 
sions. They give evidence in favor of the particular theory of 
emulsions advanced in 1917 by Harkins, Davies, and Clark. 
According to this theory the sizes of drops in an emulsion pro- 
duced by the use of a soap as an emulsifying agent depend upon 
the shapes of the molecules in the film around the drops. In 
an emulsion produced by sodium oleate, it may be assumed that 
the film contains sodium oleate, oleic acid, and oleate ions. In 
the experiments of Harkins and Keith, represented in the figure, 
the peak in the curve for sodium oleate comes at 4 microns 
(4 X 10-4 cm.). If a sodium oleate molecule has a length of 
28 X 10-8 cm., as accords with work on the thickness of films, 


; 1 
tne length of the molecule is only 750 of the radius of the drop, 
and to fit into such a surface the polar end of the molecule would 
: , 1 
need to have a cross-section with a diameter only 750 greater 


than that of the non-polar end. Now what data we have indi- 
cate a much greater difference than this, so it would seem that 
in this case the size of the drop is not entirely determined by 
the shape of the molecule of soap. 


202 COLLOIDAL BEHAVIOR 


The addition of sodium hydroxide to the partly hydrolyzed 
sodium oleate would repress the hydrolysis, and should, accord- 
ing to the theory, produce a considerable diminution in the size 
of the drops of oil in the emulsion. The figure indicates that 
when the 0.1 m sodium oleate solution was made 0.1 m in 
sodium hydrolysis, a remarkable change in the size of the drops 
was obtained, since the diameter was decreased to only one- 










05 _TEW Oeate KOR 1-1 1 | ee 
{Alt ee ee 
Bess ee 
iE bea OleatesNa OH || {—_T _ Pe thee ee ae 
Ad ode ot 
Sa dda RL 
aL Pe ee ee 







20 













Number of Drops per 100 Drops Measured 


BOTs Ren 
s/t NN ON ee 
Po MA ES 0 Oleg a ee ee 
ANE 





0 
Qcok 2° eS 405 2br as Gite aet Gem ee ey 
Size of Drops in Microns 


Fig. 18.—Relation between the size of the polar end of a molecule (as increased 
by passing from a Na soap to the K and Cs soaps), and the size of the emulsified 
oil drops; and the effect of hydrolysis on the size of the drops. 


third of that obtained when the soap was used alone. The 
- figure indicates that as the size of the polar end of the molecule 
is increased by a change from the lithium soap to the sodium, 
the potassium, and the caesium soap, the size of the emulsified 
oil drops decreases, and the peak in the curve becomes higher 
and higher, Finkle, Draper, and Hildebrand found a similar 
shift in size, but not such an increase in the height of the peak, 
in the change from a sodium to a potassium, and to a caseium 
soap. The figure shows that with potassium oleate, when its 


pti > - 


SURFACE ENERGY IN COLLOID SYSTEMS 203 


hydrolysis is repressed by potassium hydroxide, the peak lies 
higher and at smaller sizes of the oil drops than is found for 
similar emulsions with sodium hydroxide. The relative shift 
of the peak between sodium oleate with sodium hydroxide, and 
potassium oleate with potassium hydroxide, amounts to about 
one-fourth of the diameter of the drop, which is about the same 
as is found when the soaps are used alone. 

The sizes of the drops are changed by a change in the oil which 
is emulsified, and is in the direction of larger drops as the vis- 
cosity increases. ‘The addition of oleic acid is found to have a 
marked influence upon the sizes of the drops. 

Thus it is found that a repression of hydrolysis, or a change 
in the size of the atom of the metal, produces a shift in the size 
of the emulsified drops in the direction specified by the par- 
ticular form of the wedge (or truncated cone) theory of emulsions 
and the inversion of emulsions (see Fig. 6) suggested by Harkins, 
Davies, and Clark in 1917. However it is true that the sizes 
of the drops is always larger than would result if the film were 
made ‘“‘unstaggered”’ and consisted of molecules of soap alone, 
which fact was recognized when the theory was suggested. 

The form of the curve of distribution for the sizes of the drops, 
as given in the figure, seems to be much like that of the dis- 
tribution of molecular speeds according to the theory of Maxwell. 
All of the curves, except perhaps that for lithium oleate, seem to 
be of the same general form. The points plotted in the figure 
represent the actual measurements, and there has been no 
rounding off in order to improve the smoothness of the curves. 
The lower right-hand portion of a number of the curves has been 
omitted in order to avoid confusion due to the great number of 
points in this part of the figure. The area under any one curve 
is the same as that under any other. 

As has been stated, a change in the oil which is dispersed 
in an emulsion produces a remarkable change in the size of the 
drops, even when the emulsion is prepared in exactly the same 
way. Thus the most probable size for sodium oleate emulsions 
prepared by Harkins and Keith, was found to be 9.2 for stano- 
lax, 3.9u for octane, and 1.94u, for benzol or mesitylene. Table 
V gives the most probable size, that is the peak in the curve, for 
various emulsions as obtained by the use of a carefully stand- 
ardized method of preparation. 


204 COLLOIDAL BEHAVIOR 


TaBLE V.—Sizes oF Drops 1n Emuxsions or Various Oms in 0.1 m 
SOLUTIONS OF OLEATE Soars (Harkins AND KEITH) 


















































3 4 
1 2 acer Per cent 5 
Soap Added reagents of drops | (3) X (4) 
peak 
at peak 
Octane 
Lithiuiy 9}. § oe bee 4.75 7.25 34.4 
SOCIUED <0 F9Lt cee nal,  ee 3.90 9.5 36.0 
Potassium elo ee nde ae eee 2.90 IS 76 38.0 
Rabidiuma ces eae rea hee 2.50 17.00 42.5 
Caestum =. (0) ee ee et eee 1.95 yaa? 41.3 
Sodium 0.1 m sodium hydroxide 1.45 23.8 34.5 
Potassium | 0.1 M potassium hydroxide LZ (28) 33.0 
Sodium 0.1 oleic acid in oil 1.45 py Bae 39.6 
Stanolax 2 
Sodium sd" lias wee eae a 9.2 3.7 34.0 
Potassium |. 9 «legis eae eee 6.9 4.91 33.9 4 
CC@OSTUIT, 1) seat ee 4.6 6.08 28.0 
rs 
Sodium 0.1 m sodium hydroxide 5.75 4.13 24.0 4 
Sodium 0.1 m oleic acid in oil 4.6 7.83 36.0 : 
Sodium 0.1 m sodium chloride yer gs 5e22 30.0 : 
Sodium | 0.1 mM potassium iodide 6.9 4.08 ys) 4 
¥ 
i 
Benzol a 
Sodium RA MRR poo pi oe 1.94 | 19.82 38.4 i 
Mesitylene : 
z 
Sodium: J) 2.28 ae ee ae ee 1.94 a ta | 41.0 
| 








SURFACE ENERGY IN COLLOID SYSTEMS 205 


The oil drops in an emulsion carry a negative charge. The 
effect of a charge is to keep the drops apart by repulsion, and to 
decrease the surface tension by giving the drop a tendency to 
expand. Both of these effects increase the stability of an emul- 
sion or any other colloid. The potential difference at the inter- 
face between two liquids, or between a liquid and a solid, may be 
changed by a variation in the hydrogen ion concentration. 
It has been shown by Powis that the stability of dilute oil in 
water emulsions (without the presence of a soap) persists at a 
P.D. greater than 0.03 volt, but that within the range +0.03 the 
stability disappears. Different types of bacteria, for example, 
exhibit different ranges for this zone of instability. The insta- 
bility is, in general, greatest at the isoelectric point (Hardy). 

Preliminary determinations of the cataphoresis of the oil drops 
in emulsions in oleate soaps have been made for the writer by 
Professor Falk of the University of Chicago. They show that 
the oil drops carry a negative charge, and that the p.p. is about 
0.06 volt for emulsions produced by sodium, potassium and 
caesium oleates. The numerical value is considerably decreased 
by the addition of either sodium hydroxide or sodium chloride 
to the aqueous phase in which sodium oleate is the emulsiying 
agent. ) 

The changes produced in the surface energy of an interface by 
electrical charges, and by the nature and magnitude of the p.p., 
are of fundamental importance in connection with the stability of 
colloids, and will be discussed in connection with the treatment of 
colloid systems in various other chapters. 


Ture THERMODYNAMICS OF SURFACES 


The most fundamental relations in connection with surface 
energy are easily developed by the application of the principles of 
thermodynamics. 

The interfaces commonly called ‘‘surfaces’”’ are of several 
kinds: liquid-vapor, liquid-liquid, liquid-solid, and _ solid-solid. 
The thermodynamics of saturated films has been treated by 
Dupré,* Lord Rayleigh,?’ and, more thoroughly, by Gibbs‘ and by 
Einstein.28 The following treatment is somewhat different from 
that presented by any of these workers, and has some advantages. 


206 COLLOIDAL BEHAVIOR 


The surface film may be either saturated or unsaturated, and a 
part of the treatment presented will be applicable to either case. 


Let S = the entropy of the whole surface, s of unit area 

U = intrinsic energy of the total surface 

u = the intrinsic energy of unit area 

y = the free surface energy per unit area, or the surface 
tension per unit length 

c = the specific heat of the surface, where specific refers to 
unit area and not to unit mass 

A = the area of the film 

1 = the latent heat of the surface per unit area 

@ = the total heat added to the surface 


These quantities may be thought of as applying to a film of 
infinitesimal thickness, but, since this is not the true thickness for 
the actual film, they represent the surface densities. Thus U, 
as Gibbs states, denotes the excess of the energy of the actual 
mass which occupies the total volume considered, over that 
energy which it would have if on each side of the surface the 
density of energy had the same uniform value quite up to that 
surface which it has at a sensible distance from it. 


From the first law 


Ou Ou 
dQ = d(Au) — ydA = A SVaT +[A 5 ts 7) |aa 


(1) 
= CdT + IldA 
This equation is perfectly general. 
We will define c as follows: 
= 1/d0\ ie ae 
Dari) ei: ee 


Equation (1) now becomes: 
dQ = AcdT +1 dA = AC dT+ (1+ y) dA—ydA. (8) 


Let the film be saturated, and the definition of a saturated film 
will be that wu and y are functions of T only. 


d(Au) = As“ av +udA = AcdT + 


dT 
differential. (4) 


SURFACE ENERGY IN COLLOID SYSTEMS 207 


and 
c, 1 = ft) only. (5) 
A i iedt rig) eae (6) 
on dT 
u=I+y (7) 


[++y¥ =fc dT, and is a definite function of T. (8) 


From the second law 


dQ _ Ac 


dS = 7 =P 


dT + ie is an exact differential. (9) 


:(:) 











o> alae, peck din T (10) 
Combine (6) and (10), and 
yO, Ae 
Dear = op (+) (11) 
al ot _ al | dy 
imeem olee ar 
or 
Be OR 2 a OY 
ee ot ola T a 
Equation (7) now becomes 
0 
b= ee) — T (13) 
‘ ay) _ ay , ab _ al ay 
: = oan amen ieteol oT or ae. 
or 
st Ree  Astae 
Cos T (=A (14) 


Equation (1) is fundamental for saturated and unsaturated 
surfaces, and equations (13) and (14) for saturated films. 
The experimental results indicate that over moderate ranges of 


d 
temperature y is a linear function of the temperature, or oat a 


208 COLLOIDAL BEHAVIOR 


k. So long as this is true, u, the total surface energy per unit 
area, 18 independent of the temperature. This constancy of u 
makes it a much more characteristic function than either 
the free surface energy or the latent heat of the surface (1). 
Table III shows that, indeed, the total surface energy is not 
only largely independent of temperature, but is also very char- 
acteristic of any special class of compounds. In so far as y 
deviates from a linear function, wu will vary with the temperature, 
so that even a greater regularity might have been obtained by 
calculating u from data taken at corresponding states. However, 
the regularities in the values of u as they have been obtained are 
extremely striking. 


So | oN traight li hich 1 1 t Be etl 
0 long as Grp 1s a straight line, c, which is equal to dT?’ 's 


zero; or the superficial specific heat is zero, as was found by Ein- 
stein. This indicates that under this condition all of the energy 
of formation of a surface (= u or y + 1) goes into the surface in a 
potential form, and this, in turn, seems to show that the energy 
is stored up by some configuration of the surface layer. The 
fact that the total surface energy is almost independent of 
the temperature indicates that this configuration is almost the 
same for various temperatures, but it is quite likely that there 
may be some change in the relative distances of the molecules 
due to thermal expansion. 

If we now imagine that inside a liquid, a plane area of 1 sq. cm: 
exists, and then pull the liquid apart over this area, so that two 
surfaces each 1 sq. cm. in area are formed, then, as shown 
previously the increase in free energy is 2y. Now equation 
(13) shows that at the same time there is a cooling of the surface 
equal to 2/1, so that the kinetic energy of the molecules aids in 
pulling the surfaces apart. The total energy 2u involved in this 
process may be imagined to be used up (1) in giving orientation 
to the two sets of molecules on both sides of the imaginary plane, 
while still in the liquid, and (2) in pulling the two oriented surfaces 
apart. By this it is not meant that the process takes place in 
this order, but only that the end result is the same as the result 
of these two steps. While we have at present no way of deter- 
mining the relative amounts of energy involved in the two steps, 
it would seem that the amount of energy involved in (1) is rela- 





SURFACE ENERGY IN COLLOID SYSTEMS 209 


tively small, and that the greater amount is that of step (2). 
Now, if we knew the laws according to which the electromagnetic 
forces vary in the fields between and in the molecules which make 
up the surface layers, it would be possible to get a solution of our 
problem. However, since this is not known, it seems better to 
solve the converse problem, that of determining the rate at which 
these forces fall off, from the data on surface energy. 

Equation (12) indicates that if the free surface energy decreases 
with the temperature, as is usually the case, the formation of the 
surface will have a cooling effect, while if it increases with the 
temperature, as it does in the case of the two anisotropous liquids, 
ethyl p-azoxybenzoate, and ethyl p-ethoxybenzalamino-a-methyl- 
cinnamate, there will be a heating effect. In either case the first 
effect is to increase the free surface energy. It is evident that a 
contraction of an ordinary surface lowers its tension, and that the 
free surface energy of a fresh surface, between two phases which 
may exist together, is greater than that of an old one. If a sur- 
face is thought of as being formed with infinite speed, then the 
changes which follow are such as to lower the free energy. 

From thermodynamics we learn that the arrangement of the 
molecules in the surface of a liquid must be such as to make the 
free surface energy y a minimum. Now, since 


Vertes os (15) 


and when the change is not isothermal, T changes, and when the 
change is not reversible, the superficial entropy s changes, the 
condition for an isothermal reversible change in the surface is 
that there shall be a decrease in the total superficial energy wu just 
equal to that of the superficial free energy 7, so under the given 
conditions the configuration must be such as to make the total 
surface energy a minimum. 

If these equations are compared with Fig. 2, which is pri- 
marily due to Jaeger,”* it is seen that the relations indicate that the 
specific heat of the surface is not wholly independent of the 
temperature, and that it approaches zero with extreme rapidity 
as the critical temperature is approached, so the surface film 
thickens rapidly and rapidly loses the orientation of its molecules 
just before the critical temperature is reached. 


210 


COLLOIDAL BEHAVIOR 


Tue DETERMINATION OF SURFACE TENSION 


The surface tension of a liquid may be determined by any 


one of a considerable number of methods, the most widely used 
of which are known as the drop weight and the capillary height 
methods. These methods have been studied critically: the 
former by Harkins and Brown, and the latter by Richards and 
Coombs. *? 


—_ 


oO 


REFERENCES 


Books 


. Freunpuicu, H.: “Kapillarchemie,” Leipzig, 1923. 
. Wittows and Hartscuex: “Surface Tension and Surface Energy,” 


Philadelphia, 1923. 


. Dupré: ‘Theorie Mecanique de la Chaleur,’’ Paris, 1869. 
. Gipss: Scientific Papers, vol. 1, 1906. 


Papers 


. Pockets, Miss A.: Nature, 43 (1891), 437. 
. Rayeicu, Lorp: Phil. Mag., 48 (1899), 331. 
. Devaux: Papers 1903 to 1913 reviewed in Ann. Report Smithsonian 


Inst., 1913, pp. 261-73; Soc. Franc. phys., 55 (1914), 3; 67 (1914), 3. 


. Marcetin: Ann. Phys., 1 (1914), 19. 

. LaBroustTeE: Compt. rend., 158 (1914), 627. 

. Harpy: Proc. Roy. Soc., A, 86 (1911-12), 634; 88 (1913), 303-33. 

. Langmuir: Chem. Met. Eng., 15 (1916), 468. 

. Harkins, Brown and Daviss: J. Am. Chem. Soc., 39 (1917), 354; 


Harkins, Davigs and Cuark: [bid., 39 (1917), 541. 


. Harxins and Cuene: Ibid., 43 (1921), 36; Harkins, CiarK and 


Roperts: [bid., 42 (1920), 700. 


. Harkins and Roserts: [bid., 44 (1922), 653. 

. Lanemurr: [bid., 39 (1917), 1848. 

. Apam, N. K.: Proc. Roy. Soc., A, 101, 452. 

. Harxins and Ewrna: J. Am. Chem. Soc., 42 (1920), 2539; Harkins and 


FetpMaNn: Jbid., 43 (1921), 2665. 


. Trause, J.: Lieb. Ann., 265 (1891), 27. 

. Harkins: Proc. Nat. Acad. Sciences, 5 (1919), 544. 

. Lunn: J. Am. Chem. Soc., 41 (1919), 986. 

. Donnan and BarKkEr: Proc. Roy. Soc., A, 85 (1911), 557. 
. Drucker: Z. physik. Chem., 52 (1905), 641. 

. WHatmouacn: [bid., 39 (1902), 129. 

. SzyszKowsk!: Ibid., 64 (1908), 385. 

. Miner: Phil. Mag. (6), 13 (1907), 96. 


26. 
27. 
28. 
29. 


30. 


ol. 
32. 


33. 


SURFACE ENERGY IN COLLOID SYSTEMS 211 


Harkins and Ew1ne: J. Am. Chem. Soc., 41 (1919), 1977. 

Ray LeieH, Lorp: Phil. Mag. (5), 30 (1890), 461. 

Einstein: Ann. Phystk., 4 (1901), 513. 

Jancper, F. M.: Koninkl. Akad. Wetensch. Amsterdam, 17 (1914), 
329, 365, 386, 395, 405, 416, 555, 571; Z. anorg. Chem., 101 (1917), 1. 

RayYLeicH: Sct. Papers, III, 562. 

Crowss: J. Phys. Chem., 20 (1916), 408. 

Determination of Surface Tension, Review and Bibliography, Frerauson, 
Fifth Report on Colloid Chemistry, London, 1923, p. 1. Capillary 
Height Method, Ricuarps and Coomps, J. Am. Chem. Soc., 37 (1915), 
1643. Drop Weight Method for Interfaces, HARKINS and HUMPHREY, 
ibid., 38 (1916), 242. Drop Weight Method for Surfaces, and Correc- 
tion Curve, Harkins and Brown, ibid., 41 (1919), 499. 

Harkins and kine: J. Am. Chem. Soc., 41 (1919), 970. 


CHAPTER VII 
THE THEORY OF EMULSIFICATION 


By 
JoEL H. HinpEBRAND 


When two incompletely miscible liquids are mechanically 
agitated so as to disperse one of them in the other in the form of 
droplets, an amount of work must be performed upon the system 
which is equal to the product of the interfacial tension by the 
increase in surface. This work may be considerable, and accounts 
for the fact that emulsions made from two pure liquids are always 
unstable, the coalescence of the droplets upon contact liberating 
the stored energy. To stabilize an emulsion, therefore, it is 
necessary to add some third substance which is capable of produc- 
ing a film which will prevent the coalescence of the drops. The 
conditions for the stability of such a film are, in part, the same 
as those which give stability to the liquid film between the bubbles 
of a foam, or “‘emulsified gas.” 


ADSORBED FILMS 


In the first place, the work done by coalescence of the drops is 
diminished by diminishing the interfacial tension. An emulsify- 
ing agent, therefore, which considerably reduces the interfacial 
tension also reduces the tendency towards coalescence. Such an 
agent also has a further stabilizing effect upon the film separating 
two droplets similar to the effect of soap and other foaming agents 
upon the stability of the liquid film separating two bubbles in a 
foam.! This effect is brought about as follows: Whenever a 
substance lowers a surface or interfacial tension it tends to diffuse 
into the surface and to become there more concentrated than in 
the interior of the liquid.2 At a fresh surface, before diffusion 
has had time to establish the adsorbed layer, the surface tension 
is higher than at an old surface. Rayleigh has given figures for 

212 ' 


THE THEORY OF EMULSIFICATION 213 


the surface tension of soap and saponin solutions obtained by 
both dynamic and static methods, here reproduced in Table I, 
which show the great difference that may be obtained. 








TaBLe [ 
Surface tension 
Liquid 
Static _ Dynamic 
co Ais) 6 6 alt Aik Ls a Ae 75 | 75 
e020 percent sodium oleate............... 0 55 | 79 
0.25 per cent sodium oleate.................. 26 79 
2.5 per cent sodium oleate .................. 26 58 
Se LOO a GN 2 AS Re 52 73 








Accordingly, if a film of soap solution separating two bubbles 
(or drops of oil) be threatened with rupture, the fresh surface 
produced at the threatened point has a higher surface tension 
than the adjacent surface, and thus protects itself automatically 
from further injury. We may, therefore, conclude that a 
substance which is highly adsorbed at an interface, lowering 
greatly the interfacial tension, is able to act as an emulsifying agent. 

As is well known, the soaps are exceedingly effective emulsify- 
ing agents for systems composed of water with some non-polar 
or oily liquid. Donnan’ has shown that the lowering of inter- 
facial tension, with attendant effect upon the emulsifying power, 
increases with the length of the hydrocarbon chain, becoming 
very marked with sodium caprylate (containing 8 carbon atoms) 
and increasing rapidly to the higher members. 

More recent work by Langmuir‘ and by Harkins’ and co-workers 
indicates that the molecules of a soap would tend to orient in the 
interface so that the paraffin chain is in the non-polar liquid, or 
oil, while the metallic end is in the water. The insolubility of 
one end of the molecule in the one liquid and of the other end in 
the other liquid is peculiarly favorable to adsorption at the 
interface and consequent low interfacial tension. The size of 
the molecules, moreover, diminishes the effect of thermal agita- 
tion which prevents adsorption of small molecules. 


214 COLLOIDAL BEHAVIOR 


VISCOSITY 


It is evident that the effect of the emulsifying agent upon the 
viscosity of the external phase of the emulsion should exercise 
an important influence upon the stability of the emulsion, since the 
more viscous solution would drain more slowly from the film 
between two droplets, thus retarding their coalescence. High 
viscosity also diminishes the Brownian movement, which may 
otherwise be considerable in highly dispersed emulsions, and which 
promotes collisions and hence increased opportunities for coales- 
cence. The importance of viscosity for the stability of foams 
was stated by Plateau,’ and for emulsions, analogously, by 
Hillyer,’ who nevertheless showed the far greater importance of 
low surface tension, since many cases are known where high 
viscosity does not yield emulsions, while, on the other hand, 
low surface tension and excellent emulsions are produced by 
amounts of soap so small as to have but little effect upon the 
viscosity. Holmes and Child,® on the other hand, have concluded 
that for gelatin solutions a favorable viscosity 1s more important 
than low interfacial tension in yielding stable emulsions. * 


THe TypPre oF EMULSION AND THE NATURE OF THE EMULSIFYING 
AGENT 


Although the more familiar emulsions have water as the exter- 
nal, or continuous, phase, and some non-polar liquid or “‘oil”’ asthe 
internal or discontinuous phase, itis possible, as was pointed out 
by Wo. Ostwald,’ to have emulsions of the inverse type, in which 
the water is the dispersed phase. Ostwald thought that the type 
should be determined by the volume-ratio of the two liquids, and 
that only so much liquid could be dispersed as could be packed as 
spheres of approximately equal size in the other liquid (about 
74 per cent of the total volume) and that the addition of more 


*It would seem to the writer desirable to determine whether emulsions 
prepared in solutions of different viscosity might not be very differently 
dispersed during their preparation, which would affect the rate of break- 
down. It seems difficult otherwise to account for the observation of 
Holmes and Child that the most favorable viscosity was not the maximum, 
for it is evident that in an external phase of infinite viscosity there could be 
no coalescence whatever. 


THE THEORY OF EMULSIFICATION 215 


would cause it to become the external phase. We know, how- 
ever, from the previous work of Pickering,!° that as much as 99 
per cent by volume of oil can be dispersed in a soap solution. 
The dispersed oil droplets in such an emulsion distort each other 
so as to form polyhedra,!! which makes the emulsion elastic, 
since deformation increases the superficial area of these polyhedra 
and allows the interfacial tension to act as a restoring force. 
Such emulsions, like stiff mayonnaise dressing, are, therefore, 
jelly-like in properties. 

In spite of the fact that the dispersed phase may far exceed the 
other in volume, it is nevertheless easier to make the desired type 
of emulsion by using an excess of the dispersing phase at the 
outset, and doubtless, also, using a vessel wet better by this 
phase than by the inside phase. As the emulsification proceeds, 
the liquid to be dispersed may be added in larger quantities, since 
it suffices to have its volume less than that of the emulsion already 
prepared, rather than that of the external phase first used. 

It seems possible to account for the types of emulsion yielded 
by different emulsifiers most easily in cases where the latter are 
solid powders. If the solid particles are wetted preferentially by 
one liquid they go entirely into that liquid. If, however, there 
is sufficient tendency for both liquids to wet the particles, so 
that the angle of contact is finite, then the powder collects at the 
interface. This angle will rarely be 90 deg., so that the particles 
will usually be more in one liquid than in the other, and this will 
cause the interface to curve, due to the crowding of the particles 
in the better wetting liquid, which thus becomes the outer phase. 

The data at hand" seem to accord with this picture,’ although, 
as stated by Clayton,' more observations of contact angles are 
greatly to be desired. 

Where the emulsifier is a colloid, the picture may be essentially 
the same, which leads to the very useful rule of Bancroft,‘ 
that an oil-soluble colloid may emulsify water in oil, and vice versa. 
The colloid particles must have both polar and non-polar groups 
upon their surfaces, so as to be adsorbed at the interface, but they 
will tend to remain more largely in the liquid which is the better 
dispersing medium for them. 

The interfacial film may doubtless vary greatly in thickness, 
becoming sometimes a veritable skin, but there is good evidence 


216 COLLOIDAL BEHAVIOR 


that they may be so thin as to consist only of a single molecular 
layer,!® and that the molecules in this layer are oriented with their 
polar portions in water, and their non-polar portions in the oil, 
or other non-polar liquid, as previously stated. 

The réle played by the orientation of the molecules in the 
interface in determining the direction of curvature was suggested 
by Langmuir,‘ who said: 


This theory also affords an explanation of the mechanism by which 
colloids are formed. If a film of closely packed oleic acid molecules 
covers the surface of water to which sodium hydroxide has been added, 
OH groups are adsorbed by the COOH radicals, causing an expansion 
of the lower side. This results in the bulging of the film downwards 
in spots, so that it finally detaches itself in the form of particles, the 
outer surfaces of which consist of COOH groups together with the 
adsorbed OH, while the interior consists of the long hydrocarbon chains. 

The size of the colloidal particles is determined by the difference in 
size between the two ends of the molecules, just as the size of an arch 
is dependent upon the relative sizes of the two ends of the stones of 
which the arch is constructed. 


Harkins, Davies, and Clark® have also expressed the opinion 
that the natural curvative of the film is determined by the 
orientation of the molecules in the interface. They say: 


It seemed to us that the only apparent relation was that to the 
number of oleate radicals in the molecule of the protective colloid 
(sodium oleate, or magnesium oleate). Therefore, it quite possibly 
may be the orientation and the form of the molecule, together with 
adsorbed ions in the interface between the dispersoid particles (or small 
drops) and the dispersion medium, which determine the surface energy 
relations, and, therefore, the size of the drop at which it becomes stable. 
In other words, this idea is that the drop would be stable whenever the 
molecules, together with adsorbed ions, etc., in the interface fit the 
curvature of the drop. The molecules in the curved surface would 
not need to be at all of the same kind. If the molecules do not fit in 
the curved surface, the drop will not be perfectly stable, and will either 
decrease or increase in size if given time. 


It is possible to test the orientation hypothesis in a very 
striking way in the case of the soaps, where the-work of Langmuir? 
of Harkins’ and of Griffin’? justifies the assumption that at an 
interface between water and some liquid of low polarity, such as 


aah bd 


THE THEORY OF EMULSIFICATION 217 


benzol, any soap would form an interfacial film, which might 
be as little as one molecule thick. 

Now, if the polar group, in the water, occupies more space than 
is necessary for the closest packing of the hydrocarbon chain, 
the latter can be packed more closely if the film is convex on the 
water side. It is obvious that the direction and degree of curva- 
ture, if thishypothesis is correct, should vary, first, with the atomic 
volume, and, second, with the number of hydrocarbon chains at- 
tached to a single metallic atom, according to its valence. A zine 
soap, for example, with two hydrocarbon chains per atom of metal, 
crowded together in the oil phase, should tend to make the film 
convex towards the oil, while an aluminum soap, with its three 
hydrocarbon chains in the oil, should give still more curvature 
towards the water and hence relatively stable emulsions of water 
in oil. 

The relative sizes of the metallic atoms in the various soaps 
may be inferred from the atomic volumes of the metals, and from 
their atomic diameters in the free state and in compounds. 
Values are given in Table II. The atomic diameters are accord- 
ing to Hull,!® Bragg,'’ and Richards,!* respectively. Of course, 
hydration may modify the effective atomic domain, but since, for 
example, the hydration of silver ion can hardly be as great as that 
of sodium ion, this factor may be expected to increase rather than 
to oppose the effect of the differences evident in Table II. 

Application of the theory to these figures would indicate that the 
ability of soapsof Ca, K, Na and Ag toemulsify oil in water would 
decrease in the order given; or, viewed from the other angle, their 
ability to emulsify water in oil should zncrease in this order; that 
the soaps of the divalent metals, Ca, Mg and Zn, should have 
much less ability to emulsify oil in water or much greater ability 
to emulsify water in oil, and, further, that they should vary in 
these respects in the order given: The soaps of the trivalent 
metals Al and Fe, should exhibit the greatest tendency to emul- 
sify water in oil. The values in the several columns for the rela- 
tive size of Fe and Al atoms do not all agree, so that the relative 
emulsifying powers are not definitely indicated by these figures. 
In general, the values for the atomic diameters of the free metals 
are most reliable, since the values for the atomic diameters in 
compounds involve two variables which require some further 


218 COLLOIDAL BEHAVIOR 


consideration for their determination. On the other hand, there 
may be doubt that the elementary atom with its electrons occu- 
pies the same domain as does the atomic kernel in the compounds. 
However, there are enough uncertain factors involved to deter us 
from attempting to make any very fine distinctions on the basis 
of existing figures for atomic diameters. 


TaBLE IJ.—RELATIVE Si1zES oF ATOMS 














Atomic diameters 
In metal In halides Atomic 
volumes 
Hull Bragg Richards 

Ca 4i7} 3.80 70.6 
K Se 4.15 3.46 (in KC) 45.3 
Na oe 3.55 2.85 (in NaCl) | 22.9 
Ag QeS7 3,55 eee ee | 10.3 
Ca 3.93 3.40 — (4. eee 12.6 
Mg ee 2.85, 1 See eee 7.0 
Zn 267 2 .65°° - 4 eee 4.6 
Al 2.86 2.70. | eee 3.4 
Fe 2.48 2.80...) eee Jen 











There have been known for some time many facts which 
accord with this theory of the emulsifying powers of soaps. 
The soaps of the alkali metals are very effective in stabilizing oil 
in water emulsions, K soaps being more effective than Na soaps;’® 
magnesium soaps emulsify water in oil;?° salts of the trivalent 
metals, Al and Fe, are especially effective in reversing oil in water 
emulsions stabilized by soaps of the alkali metal.?!_ Emulsions 
of oil in water stabilized by sodium oleate are reversed by adding 
magnesium, aluminum, ferrous, or ferric salts in amounts chemi- 
cally equivalent to the sodium oleate used." 

Finkle, Draper, and Hildebrand have determined not onty 
the type of emulsion but their relative stabilities, using stearates 
and oleates of the metals listed in Table II. The results obtained 





THE THEORY OF EMULSIFICATION 219 


for a typical series are shown in Table III, and are seen to be in 
close accord with the theory. 








TaBLeE III 
Oleate of Dispersed phase Approximate life of 
emulsion 
Cs | Benzene 8 weeks 
K | Benzene 8 weeks 
Na Benzene 6 weeks 
Ca Water 1 hour 
Ag Water 1 day 
Mg Water 2 days 
Zn Water 2+ days 
Al Water 7 days 
Fe Water | 10 days 








The results in these cases are also in accord with the rule that 
the external phase is the one which is the better solvent for the 
emulsifier. The soaps of the alkali metals are more soluble in 
water, the others more soluble in the non-polar liquid. The soaps 
of Fe and Al have more symmetrical and less polar molecules, and 
would, therefore, be expected to dissolve best in solvents of 
low polarity, while the alkali metal soaps, consisting of a single 
chain with a highly polar end, cannot dissolve considerably in 
non-polar liquids, but can, on the other hand, form clusters with 
the hydrocarbon chains in the interior, which can then dissolve 
“colloidally’’ in water. 

If the orientation theory is correct, the different degrees of 
curvature natural to the films of different soaps should yield 
drops of different size. The authors just mentioned compared 
the drop sizes in emulsions of Na, K, and Cs soaps, respec- 
tively, and found the following sizes for the drops present in 
largest number in the three cases: Na soap, 0.005 mm.; K soap, 
0.004 mm.; Cs soap,0.0025mm. The same relative order, though 
with different numerical values, was obtained in other series. 


CRACKING OF EMULSIONS 


The cracking of emulsions can be sought by destroying the 
conditions for stability. Thus, an emulsion of an oil in water 


220 COLLOIDAL BEHAVIOR 


stabilized by the soap of an alkali metal can be cracked by 
adding the equivalent amount of heavy metal salt.21_ Water- 
in-oil emulsions can be treated by alkali soaps,?* sodium carbon- 
ate,24 sodium salts of sulfonated oils, and the like. Any reagent 
which destroys the emulsifier will also promote demulsification. 

Electrical methods of demulsification are extensively employed 
and are effective, due to the effect of an electric charge upon the 
interfacial tension.2> The relation between the charge on a 
sphere «, its radius r, and the increase in surface tension due 
to the charge Ay, is given by the equation: 


me 
Bat. 
Thus, if Ay = —20 dynes, andr = 10-‘ cm., e = 0.32 c.g.s. units, 
or 95 volts. An electric field of sufficient magnitude, therefore, 
can exert enormous effects upon the surfaces of the droplets and 
destroy the stabilizing film. 


REFERENCES 


1. RayuercH: Proc. Roy. Soc., 47 (1890), 281. 
2. Grpss: Scientific Papers, 1 (1906), 219. 

For derivation of the quantitative relationship between adsorption, 
surface tension, and free energy, see Lewis and Ranpawu: “Thermo- 
dynamics and the Free Energy of Chemical Substances,” McGraw- 
Hill Book Co., 1923, p. 249. The present author has discussed this 
relation in connection with deviations from the ideal solution laws; 
Hiwpesranp: ‘Solubility,’ American Chemical Society Monograph 
Series (1924), Chap. XVIII. 

3. Donnan: Z. physik. Chem., 31 (1899), 42. 
Donnan and Ports: Kolloid Z., 4 (1910), 208. 

4. Lanemutr: Chem. Met. Eng., 15 (1916), 468; J. Am. Chem. Soc., 39 
(1917), 1848. 

5. Harkins, Daviss and Cirark: J. Am. Chem. Soc., 39 (1917), 354, 541. 

6. PuaTEAu: Pogg. Ann., 141 (1870), 44. 

7. Hrtiyer: J. Am. Chem. Soc., 25 (1903), 513. 

8. Hotmss and Cuitp: J. Am. Chem. Soc., 42 (1920), 2049. 

9. OstwaLp: Kolloid Z., 6 (1910), 103; 7 (1910), 64. 

10. Pickertna: J. Am. Chem. Soc., 91 (1907), 2002. 

11. Bancrort: J. Phys. Chem., 16 (1912), 179. 

12. Picxertne: J. Chem. Soc., 91 (1907), 2010; Kolloid Z., 7 (1910), 11; 
Suepparp: J. Phys. Chem., 23 (1919), 634; ScutaEprER: J. Chem. Soc., 
113, (1918), 522; Moorn: J. Am. Chem. Soc., 41 (1919), 940. 

13. Finke, Draper and HILDEBRAND: J. Am. Chem. Soc., 45 (1923), 2780. 


- 


14. 


15. 


16. 
ris 
18. 
19. 


20. 
21. 
22. 
23. 
24. 
25. 


THE THEORY OF EMULSIFICATION 221 


Cuayton: “The Theory of Emulsions and Emulsification,” P. Blakis- 
ton’s Son and Co., 1923. 

Apams: Proc. Roy. Soc., London, 99A (1921), 336; 101A (1922), 452. 
GriFFin: J. Am. Chem. Soc., 45 (1923), 1648. 

Huu: Proc. A. I. LE. E., 38 (1919), 1171; Science, 52 (1921), 227. 
Braae: Phil. Mag., 6, 40 (1920), 169. 

Ricuarps, T. W.: J. Am. Chem. Soc., 45 (1923), 422. 

Neunier and Maury: Collegium (1910), 277; Chem. Zentr. (1910), 
II, 1416. 

Newman: J. Phys. Chem., 18 (1914), 34. 

BHaTtTnaGar: J. Am. Chem. Soc., 119 (1921), 61, 1760. 

GrirFin: J. Am. Chem. Soc., 45 (1923), 1648. 

Parsons and Wixson: J. Ind. Eng. Chem., 13 (1921), 1116. 
MatTHEews and Crossy: J. Phys. Chem., 20 (1916), 407. 

Cf. Eppy, W. G, and H. C.: J. Ind. Eng. Chem., 13 (1921), 1016. 


CHAPTER VIII 
EMULSIONS AND FOAMS* 


By 
Harry N. HoLuMeEs 


Emulsions are dispersions of one liquid in another liquid. 
Strictly speaking, the drops or globules should be of colloidal 
dimensions, yet much coarser dispersions are often included 
in the term emulsions. Two mutually insoluble liquids may 
be shaken, beaten, or ground together to form emulsions, but 
they soon separate into two layers of the original liquids. Such 
emulsions are stable only if the dispersed phase does not exceed 
1 or 2 per cent of the total volume. Condensed water from an 
engine often carries an extremely small amount of lubricating 
oil thoroughly emulsified. 

To prepare a similar emulsion, one may pour 10 ce. of a 1 
per cent solution of any suitable oil in acetone (or alcohol) into 
1,000 cc. of water. The author has at hand a comparatively 
stable emulsion of this type prepared nine years ago. Distilla- 
tions from mixtures often produce annoying emulsions. 

The stability of these extremely dilute emulsions of ‘‘oil in 
water’’ is probably due very largely to the negative charge, of 
the order of 0.05 volts (Lewis!), carried by the drops. In this 
respect they resemble suspensions of solids in liquid. There may 
be resemblance in another respect, too. When drops are suffi- 
ciently small, surface forces give them the rigidity of solids. In 
this connection it might be remarked that if one melted a solid 
fat and dispersed it in hot water, an emulsion would result, but, 
on sufficient cooling, this emulsion would change to a suspension 
of a solid in a liquid. 


*Some of the material in this chapter was taken, by permission of John 
Wiley & Sons, Inc., from the Laboratory Manual of Colloid Chemistry by 
the Author. 

222 


EMULSIONS AND FOAMS 223 


With the exception indicated above, stable emulsions of two 
pure liquids cannot be prepared. A third substance, usually 
colloidal, is necessary to stabilize emulsions. This is often 
present as an unsuspected impurity, or it may be added purposely. 
The exact manner in which the emulsifying agent functions 
is still disputed. The various theories are presented in the 
following section. 


EMULSION THEORIES 


Quincke,? and later Donnan and Potts,’ held that interfacial 
tension lowering was a very important factor in stabilizing emul- 
sions. This view receives much experimental support. The 
alkaline soaps are, perhaps, the most commonly used emulsifying 
agents and, as a class, they produce marked lowering of the 
surface tension of water. Donnan and Potts showed that notice- 
able emulsifying power and surface tension lowering in the fatty 
acid series begins with the alkaline laurates. These two prop- 
erties become more marked with increase in molecular weight, 
as do colloid properties in general. 

Water has a high surface tension, which naturally tends to 
pull all the water of an emulsion into one large drop, or layer, in 
order to expose the least possible surface. Any substance that 
lowers this tension must weaken this layering tendency, and thus 
stabilize an emulsion. 

The most generally accepted theory is the adsorption film 
theory advanced by Bancroft.4 This is founded upon Gibb’s 
statement that any substance which lowers the surface tension of 
a liquid must concentrate at surfaces or interfaces. By such con- 
centration, a film forms around each drop, thus interfering with 
coalescence of drops and breaking of the emulsion Hillyer,® in 
his classic discussion of soaps as emulsifying agents, stresses 
the primary importance of low interfacial tension as well as the 
high superficial viscosity of certain emulsion films. Bancroft 
considers that the best film should be tough and elastic. R. E. 
Wilson® holds that it is really a plastic solid. 

Briggs’ insists that if the emulsifying agent is peptized too 
well by the continuous phase the adsorption film will not be 
formed; consequently, addition of a mild flocculating agent must 


224 COLLOIDAL BEHAVIOR 


be helpful in some instances. Thus a 1 per cent solution of 
hydrous ferric oxide was found to be a poor emulsifying agent, 
while addition of 1 g. of pure sodium chloride to 40 cc. of this 
solution made possible a coarse dispersion of 10 cc. of benzene 
in the solution. A higher concentration of salt increased the 
stability of the emulsion. Sodium sulfate, however, was too 
strong a coagulant, precipitating the ferric hydroxide in coarse 
flocks. 

The author has observed that if large drops of water are allowed 
to flow slowly from a pipette into a 2 per cent solution of gum 
dammar, visible adsorption films form in a fraction of a minute. 
When these large drops are rolled around, the films wrinkle and 
show some toughness and elasticity. 

The following is quoted from a pamphlet issued by the Sharples 
Specialty Company. 


When crude corn oil is agitated with hot water, a thick, white emulsion 
forms. When this emulsion is passed through the Sharples Super- 
Centrifuge at a moderate rate of flow, some of the oil separates and is 
discharged in a highly emulsified condition. 

The oil separated in this way will not form a stable emulsion when 
again agitated with hot water, because the emulsifying agent has been 
extracted. But, when the discharged emulsion is broken by any 
chemical means, the separated oil is easily emulsified again. 


This indicates a concentration of the emulsifying agent at the 
oil-water interface. Briggs® observed a removal of soap by the 
cream from an emulsion—another indication of concentration 
at interfaces. Martin Fischer and Hooker? believe that, in 
general, highly hydrated colloids are the best emulsifying agents, 
because they “‘bind’’ all the water. If more water is present 
than can be bound by the colloid, the emulsion is not stable. 
As support for this hydration theory, they instance the good 
emulsifying properties of such hydrated colloids as the alkali 
soaps, gelatin, gum arabic, albumin, and acid casein (or alkali 
casein). Neutral casein is a poor emulsifying agent, because 
only slightly hydrated, while either acid casein or alkali casein 
is highly hydrated and a good emulsifying agent. 

Fischer also instances mayonnaise as an example of the use of a 
hydrated colloid as emulsifying agent. The hydrated proteins 


EMULSIONS AND FOAMS 225 


of the yolk function here. In fact, egg yolk itself is an emulsion 
of over 30 per cent fat in hydrated proteins. 

Pickering’? prepared very good emulsions of oil in water by 
the use of a finely divided basic ferrous sulfate, freshly precipitated, 
as emulsifying agent. Basic cupric sulfate served as well. This 
is startling, because any films surrounding the oil drops were 
composed of small discrete solid particles, probably non-crystal- 
line. Pickering concluded that all emulsion films had a similar 
structure and gave surface tension a minor rank. In this 
he went too far. Pickering’s'! name is always associated in 
emulsion history with his 99 per cent oil emulsion with soap water 
as the continuous phase. 

Harkins, Davies, and Clark”? think that the best emulsifying 
agents have long molecules with a polar active group at one 
end of the molecule (polar groups such as -COOH, —SO3H, and 
—COOR). Langmuir’ believes that the hydrocarbon part of the 
molecule strikes inward into the fatty globules, while the -COOH, 
—SO3H, and similar groups are outside in the water phase. This 
orientation theory is quite evidently applicable to the soaps, but 
certainly does not apply to Pickering’s emulsions. 

The author regards an interfacial film as an equilibrium product 
resulting from the peptizing action on the one side and the pre- 
cipitating action on the other. If the film material be swollen 
with liquid on one side, it will be more elastic. Hildebrand offers 
the suggestion that the volume of one or the other end of a 
long molecule (such as a soap) really determines whether the 
surface shall be convex or concave toward a given liquid—in other 
words, determines whether the oil or water shall be the dispersed 
phase. Harkins, Davies, and Clark offered a somewhat similar 
suggestion a few years earlier. They contended that in a 
calcium soap, for example, the two oleate radicals (or stearate, 
etc.) in the soap chain produced a wedge shape just the opposite 
of the shape of the sodium oleate chain. Hence, the inversion 
in type. 


Two TyprrEs oF EMULSIONS 


The more common emulsions are dispersions of ‘‘oil in water.”’ 
By “‘oil’’ is meant any liquid not miscible with water. In 1910, 
Wo. Ostwald first drew general attention to another type, “water- 


226 COLLOIDAL BEHAVIOR 


in-oil’’? emulsions, in which oil is the continuous phase and water 
the dispersed drops. 

The determination of which liquid is to form the dispersed 
phase depends upon the choice of the emulsifying agent. Alkali 
soaps always give emulsions of the usual oil-in-water type, while 
heavy metal soaps yield the less usual water-in-oil type. It is 
significant that alkali soaps are soluble in, or peptized by, water 
and are usually far less soluble in the other liquid. Heavy metal 
soaps are usually less soluble in water than in the other liquid 
chosen. 

Out of such observations came a generalrule. If the emulsify- 
ing agent is more readily peptized or even more readily wetted by 


water than by oil, the oil-in-water type of emulsion results, but if 


the emulsifying agent is more readily peptized or wetted by oil 
than by water, the water-in-oil type of emulsion results. 

Schlaepfer! was able to disperse 70 per cent (by volume) of 
water in 30 per cent of kerosene, using soot as the emulsifying 
agent, because the soot was more readily wetted by oil than by 
water. W. E. Moore?® did the same thing, using carbon, oil, 
and aqueous solutions of ammonium chloride. 

Clowes!* considers that the determination of phase depends 
upon the convex or concave bending of the liquid interface as 
influenced by surface tension lowering, caused by the emulsifying 
agent. He added a calcium salt to an emulsion of oil in water 
stabilized by sodium oleate and changed the type. Of course, 
a calcium soap was formed by double decomposition, and it 
favored formation of a water-in-oil emulsion. Clowes states 
that when the equivalent ratios of Ca: Na were 1 :4, the opposing 
effects were balanced and neither type was formed. Clowes 
holds that the adsorption of Ca ions or Na ions by the film is a 
vital factor in determination of phase. 

Bhatnagar!” goes so far as to say that: 


All emulsifying agents having an excess of negative ions on them and 
wetted by water will yield oil-in-water emulsions, while those having an 
excess of adsorbed positive ions and wetted by oil will give water-in-oil 
emulsions. 


Clayton,!8 in his invaluable book on “The Theory of Emul- 
sions,” expresses the belief that “before much further advance 


oe 


EMULSIONS AND FOAMS 227 


can be made towards a general theory of emulsions attention must 
be paid to the wetting of various emulsifiers by different liquids,”’ 
and that the important physical factor to be studied is that of 
the angle of contact between liquid and solid. 

The relative volumes and order of addition of the two liquids 
in an emulsion have some influence on the type. Clayton!® 
in reporting on the manufacture of margarine remarks that, if 
oil is slowly fed into milk in bulk, with agitation, a stable oil-in- 
water emulsion results; but if milk is fed very slowly into oil 
in bulk, a very unstable system of water in oil results. 

The use of finely divided solids as emulsifying agents has been 
studied very thoroughly by Bechold, Dede, and Reiner.”° 

Holmes and Cameron”! found gum dammar, in many respects, 
the best emulsifying agent for the preparation of the water-in-oil 
type of emulsion. They also found cellulose nitrate, peptized by 
such solvents as amyl acetate, very useful in dispersing water or 
glycerol. 

A partial list of the two types of emulsifying agents follows. 
Of course, a great many more might be added if desired. 


EMULSIFYING AGENTS 


For oil-in-water emulsions: For water-in-oil emulsions: 
Sodium oleate Gum dammar 
Other alkali soaps Calcium oleate 
Gelatin Other heavy metal soaps 
Saponin Lanolin 
Albumin Rosin 
Lecithin Rubber 
Casein (acid or alkaline) Cellulose nitrate 


How To RECOGNIZE EMULSION TYPES 


The drop-dilution method of testing emulsions, as described by 
Briggs,2? is the most generally useful. A drop of the emulsion in 
question is placed upon “‘water.’’ If it mixes readily with the 
water, the emulsion is a dispersion of oil in water. If placed 
upon “oil,” such a drop would not spread readily because the 
oil drops in the emulsion would be separated from the bulk oil 
by the surrounding phase, water. The other type of emulsion 
would act in the opposite way. In general, when the drop mixes 
readily the continuous phase is the same as the bulk liquid. 


228 COLLOIDAL BEHAVIOR 


The dye-spreading method of Robertson” is often useful. A 
few minute particles of an oil-soluble dye (such as Sudan IIT) 
are sifted over the emulsion. If the color spreads, oil must be 
the continuous phase. If oil were dispersed, it is obvious that the 
dye could not jump from drop to drop. 

The differences in conductivity for heat and electricity are 
evident when the continuous phase is oil .and when it is water. 
Differences in the splash sounds when the two types of emulsion 
are shaken in a bottle may also be used in recognizing type. 


MAKING AND BREAKING EMULSIONS 


Of course, the emulsifying agent must first be dispersed in the 
liquid to be made the continuous phase. It is the general rule 
that the liquid to be dispersed should be added slowly with 
constant agitation. Briggs, however, believes in the superior 
merit of intermittent shaking. Grinding is favored by pharma- 
cists who often desire to make very viscous emulsions. Martin 
Fischer?4 advocated a rotating cone which could be forced very 
close to a casing, so that a smearing action could tear globules 
into smaller drops. This principle is used in the Premier Mill 
now being placed on the market. Briggs’?> homogenizer is a 
simple but effective laboratory device. 

The housewife is familiar with the use of an egg beater in 
whipping up a mayonnaise. Quite as simple in principle is 
shaking by hand or by machine. In this case additions of oil or 
water must be intermittent. There is need for a method of 
removing the emulsified drops as fast as formed, suggests Clayton. 
The increase in viscosity with increasing richness of an emulsion 
interferes with free splashing and smashing of the liquid to be 
dispersed. The volume of free air in the splash bottle may 
affect the method of shaking, due to concentration of the 
emulsifying agent. at bubble surfaces. 

In breaking an emulsion the important thing is to change the 
emulsifying agent chemically or physically so that it is no longer 
effective. Adding an acid to an emulsion stabilized by sodium 
oleate changes the agent into oleic acid, which has no emulsifying 
properties. Emulsions are least stable when brought near the 
inversion point, that is, near the point at which they change type. 


ee a _ 


EMULSIONS AND FOAMS 229 


Addition of an emulsifying agent of opposite type is often 
effective in breaking. ‘Thus,a calcium soap antagonizes a sodium 
soap. Sometimes addition of the dispersed liquid in bulk has a 
breaking effect. Coalescence of drops is often secured by the 
addition of polyvalent ions opposite in charge to the drops. 
Coalescence by passage of an electric current was patented by 
Cottrell?® with a view to removing the objectionable water drops 
from certain petroleums. Some emulsions can be “salted out.” 
This may mean the dehydration of the swollen films. The centri- 
fuge not only separates creams, but in some cases breaks emul- 
sions. Heating under pressure has been known to break certain 
petroleum emulsions. The emulsifying agent in oil field emul- 
sions (water in oil) is generally an asphaltic material. Phenol, 
soluble in both oil and water, has been used to carry hydrogen 
ions into the water drops, thus neutralizing the negative charge 
on the drops. A mineral acid, soluble in water, tends to break 
emulsions of oil-in-water, while some organic acid, soluble in oil, 
tends to break water-in-oil emulsions. 


CREAMING 


Cream rises to the top in milk because the fat globules are 
lighter than the watery portion of milk. The centrifuge accen- 
tuates this gravity difference and so is used as a cream separator. 
The larger drops rise more readily than smaller ones. This 
difference is carried to the extreme in homogenized milk which 
never creams. The fat globules have been reduced to one-tenth 
their usual size. 

It is evident that when the dispersed phase is heavier than the 
continuous phase, the cream will sink. An emulsion of carbon 
tetrachloride dispersed in water by sodium oleate creams 
upwards, while an emulsion of carbon tetrachloride dispersed in 
water by gum dammar creams downwards. 

The obvious inference is that an emulsion made up of two 
phases of the same density will not cream. This is true to a 
degree. Temperature changes affect the densities of the two 
liquids differently, however, and the original condition might not 
continue. 


230 COLLOIDAL BEHAVIOR 
TRANSPARENT AND CHROMATIC EMULSIONS 


Usually, when two transparent liquids are emulsified, a milky- 
white mixture results, Olive oil shaken with water illustrates 
this; yet transparent emulsions can readily be prepared. ‘Trans- 
parency depends upon the relative indices of refraction of the two 
liquid phases. If both phases have the same index, there will 
be neither reflection nor refraction, and the system will appear 
homogeneous and entirely transparent. Glycerol dispersed in a 
2 per cent solution of calcium oleate in carbon tetrachloride 
yields a fairly transparent emulsion. Gum dammar could be 
substituted for the calcium oleate. 

Holmes and Cameron?’ dispersed glycerol in amyl acetate 
containing about 2 per cent of cellulose nitrate (11.04 per cent 
nitrogen). The emulsion was milky in appearance. They 
then gradually added, with shaking, a considerable volume of 
carbon disulfide, and the emulsion became nearly transparent. 
The explanation is simple enough. Carbon disulfide dissolved 
in the continuous phase, amyl acetate, and raised its index of 
refraction to equality with that of the glycerol. On further 
addition of carbon disulfide, beautiful color changes appeared. 
The whole chromatic scale of colors was brought out and was 
reversed by additions of amyl acetate, which, of course, lowered 
the index of refraction of the continuous phase. Benzol may 
be substituted for the carbon disulfide, although the results are 
not so striking, rubber for the cellulose nitrate, and water solu- 
tions for the glycerol. 

To secure such structural colors (a sort of Christiansen effect), 
it is necessary to have two mutually soluble liquids for the con- 
tinuous phase, one of high refractive index and high optical 
dispersive power (as a prism disperses light). Carbon disulfide, 
benzene, and a concentrated aqueous solution of potassium iodide 
supply high optical dispersive power. Size of drops has little 
to do with the phenomenon. The gradual change in optical 
dispersive power is vital. 


FROTH AND Foam 


There is a close resemblance between emulsions and foam. 
The one is a dispersion of liquid in liquid and the other a disper- 


EMULSIONS AND FOAMS 231 


sion of gasin liquid. Adsorption films act as emulsifying agents, 
and absorption films may surround the gas bubbles in foam. 

Miss Benson” first demonstrated that a froth of aqueous 
amyl alcohol showed a higher concentration than did the solution 
beneath. Kenrick*® shook 0.08 g. of methyl violet in 300 ce. of 
water and secured a good froth. After removing this froth he 
broke it with a drop of ether, and, by color comparisons after 
diluting with water, he proved that the dye had concentrated in 
the froth films. Adsorption at a liquid-gas interface is the 
same sort of thing as adsorption at a liquid-liquid interface. 

The saponins are remarkable frothers, better even than the 
alkali soaps. ‘These soaps lower surface tension and so concen- 
trate at interfaces, but the saponins do not lower surface tension 
greatly. Their frothing power is due largely to the high super- 
ficial viscosity of saponin films. To use R. E, Wilson’s phrase, 
these films are “‘ plastic solids.” 

Flotation froths are of this type. Pulverized sulfide ores, 
for example, are beaten in water carrying a little oil. Since 
the gangue is preferentially wetted by water, this portion of the 
ore sinks, while the valuable sulfide, being preferentially wetted 
by oil, attaches itself to the oily froth and ‘‘floats.’”?’ The very 
temporary froth is given a high superficial viscosity by the finely 
ground sulfide and is thus stabilized. Of course, the froth is 
removed, broken by jets of water, and the sulfide recovered. 

A famous fire-extinguishing mixture contains soda and alum 
to furnish carbon dioxide, and an extract of licorice to give 
stability to the foam. Bancroft, in his ‘‘ Applied Colloid Chem- 
istry,” calls attention to the fact that violent shaking of a rennet 
solution destroys most of its power to coagulate milk. The 
enzyme concentrates in the froth. On the other hand, shaking 
does not inactivate a rennet solution carrying some saponin, 
because the latter is preferentially adsorbed at the interfaces 
and thus keeps the rennet from being coagulated. 

Salts cause foaming (or “priming”’) in boiler water, but not 
because the surface tension is lowered. Bancroft*® insists that 
either increase or decrease in surface tension is sufficient to 
cause foaming. 


232 COLLOIDAL BEHAVIOR 


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REFERENCES 


. Kolloid-Z., 4 (1909), 211. 

. Ann. (Pogg.), 189 (1870), 1-89; Ann. (Wied.), 35 (1888), 571-580. 
. Kolloid-Z., 7 (1910), 208-214. 

. J. Phys. Chem., 17 (1913), 514-18; J. Phys. Chem., 19 (1915), 275. 


J. Am. Chem. Soc., 25 (1903), 513. 
Chem. Met. Eng., 24 (1921), 825. 


. J. Ind. Eng. Chem., 13 (1921), 1008. 
. J. Phys. Chem., 19 (1915), 210: 
. Kolloid-Z., 18 (1910), 129; ‘‘Fats and Fatty Degeneration,” John 


Wiley & Sons, New York, 1917. 


. Kolloid-Z., T (1910), 11-16; J. Chem. Soc., 91 (1907), 2010. 


. Chem. Soc., 91 (1907), 2002. 

. Am. Chem. Soc., 39 (1917), 541-596 
. Am. Chem. Soc., 34 (1917), 1848. 

. Chem. Soc., 113 (1918), 522. 

. Am. Chem. Soc., 41 (1919), 940. 

. Phys. Chem., 20 (1916), 407-451. 

. Chem. Soc., 120 (1921), 1768. 


i ae Bite hn ie ee! 


. “The Theory of Emulsions,’ Blakiston’s Son & Co., Philadelphia, 


1923. 


. J. Soc. Chem. Ind., 36 (1917), 1205. 

. Kolloid-Z., 28 (1921), 6-19. 

. U.S. Pat. 1429480 (1922). 

. J. Phys. Chem., 18 (1914), 34. 

. Kolloid-Z., T (1910), 7-10. 

. ‘Fats and Fatty Degeneration,” John Wiley & Sons, 1917. 
. J. Phys. Chem., 19 (1916), 228. 

. U.S. Pat. 287115 (1911): 

. J. Am. Chem. Soc., 44 (1922), 71. 

. J. Phys. Chem., 7 (1903), 582. 

. J. Phys. Chem., 16 (1912), 517. 

. “Applied Colloid Chemistry,’”? McGraw-Hill Book Co., 1921. 


- AT Senate pinback 





CHAPTER IX 
ADSORPTION IN COLLOID SYSTEMS 


By 
Leonor MICHAELIS 


In the classical development of the phase rule the phenomenon 
is, in general, ignored that the surfaces of the phases may have 
different composition from the main portion of the phase, not- 
withstanding the existence of chemical equilibrium. Complete 
homogeneity within each phase is a necessary assumption in 
order to define it as a true phase. This definition requires that 
each phase has a uniform composition even to the very point of 
contact with the second phase. It is with the correction of 
this wrong assumption that the phenomenon of adsorption is 
chiefly concerned. The necessity for this correction was clearly 
recognized and expressly formulated by Willard Gibbs,! the 
originator of the phase rule. Quite independently, though at a 
later date, J. J. Thomson? deduced the same relation. The 
Gibbs theorem is stated 


where uw is the excess in concentration, in the boundary layer, 
of a substance dissolved in a phase or present in the gaseous 
form over and above its concentration c within the phase. The 
upper limit of concentration is thus c+ yu. Further, o is the 
surface tension of the boundary layer and P the osmotic pressure 
of the solute (or alternatively the gas pressure). If one accepts 
the validity of van’t Hoff’s law for dilute solutions, then 

apie 

(hel Ge 
where R is the gas constant and 7’ the absolute temperature, and 
the former equation may be written 

an c do 

maied PTde 
233 


234 COLLOIDAL BEHAVIOR 


At a is negative, that is, if the dissolved substance lowers 


the surface tension of the solvent, and, indeed, the higher the 
concentration the greater the lowering, the adsorption becomes 
positive, and the solute is in higher concentration in the surface 
layer. The alcohols, esters, and many other organic non-electro- 
lytes in aqueous solution are in this sense surface active or capillary 
active substances. It also appears to be true that, within an 
homologous series of similarly constituted substances, the sur- 
face activity increases rapidly with each increase in the carbon 
chain, as was stated by Traube. Such substances are, therefore, 
a suitable starting point in the study of adsorption. 

The theorem of Gibbs is based on thermodynamics and cannot, 
indeed, be questioned. But we may ask what can be accom- 
plished with it, whether we are in a position to draw conclusions 
from it which experiment can confirm, and what degree of 
accuracy we may expect from it. 


THE PRACTICAL SIGNIFICANCE OF THE GIBBS THEOREM 


In the first place, can one evolve a definition of adsorption 
by original reasoning from the Gibbs theorem? We may under- 
stand by adsorption the phenomenon that takes place when an 
enrichment of a dissolved substance in the surface layer 
is to be found. The enriched substance we may call the 
adsorbate (after Taylor) or the adsorbed phase; the other 
phase the adsorbent or the adsorber. This definition is use- 
ful, but it leads to a peculiar consequence. Thus, when a 
solution borders a vacuum, or, more exactly, when it borders its 
vapor (for equilibrium is demanded) which, in comparison 
with the liquid phase, has almost no mass, the surface of the 
solution enriches itself with a capillary active substance, as is 
required by the Gibbs theorem. In this case the vacuum 
is the adsorbent. This somewhat strange deduction from the 
definition can be accepted, as it is based on a purely formal defini- 
tion. Should it become desirable in some special case to differ- 
entiate, this could be designated as apparent adsorption® in 
contradistinction to true adsorption. Gibbs’ theory may, of 
course, be considered only as a law holding under ideal conditions 





ADSORPTION IN COLLOID SYSTEMS 235 


and one which is realized experimentally in few cases. Adsorp- 
tion is measured at the concentration u, which refers only to the 
outer limiting surface. The concentration of the limiting surface 
does not, however, necessarily fall suddenly to that of the remain- 
ing solution, but sometimes gradually, even though the concen- 
tration gradient is steep. Analytically, however, we cannot 
determine the concentration of the limiting layer, but only the 
total loss produced in the solution by adsorption. In the case of 
a monomolecular adsorption layer, the conception of the ‘“‘con- 
centration’ of the surface layer becomes still more doubtful. 
The factor uw is further represented as a function of quantities 
which we can only partially measure. 7Z' and R are, indeed, 
simple, but the determination of the magnitude of the surface 
tension presents the greatest difficulties. We can measure 
the surface tension only against air or, in special cases, 
against a second liquid immiscible with the first; unfortunately, 
therefore, only in such cases as are not adapted to analytical 
adsorption experiments. ‘The surface tension of a liquid against 
a solid adsorbent which may be used in powdered form, as char- 
coal, is not measurable. The only method by which one may 
determine qualitatively whether a substance dissolved in water 
lowers the surface tension against charcoal is merely the investi- 
gation of its ability to adsorb this substance. Experience 
teaches us naturally that, in a series of cases, surface activity 
proceeds parallel to adsorption by a solid adsorbent (Freund- 
lich’s rule). Thus, for example, the alcohols or esters of an 
homologous series are increasingly better adsorbed by charcoal in 
the same degree as their surface tension against air becomes greater. 

But there are many exceptions to this rule. In the first place 
there are substances which increase the surface tension of water 
against air and still are adsorbed by charcoal (sugar). In the 
second place, this parallelism, so far as it exists, is manifest only 
in the case of adsorption by charcoal. Thus, for example, there 
is no surface-active non-electrolyte which is adsorbed in the 
slightest degree by kaolin or by ferric hydroxide.’ In these cases 
the surface tension against air is no criterion for the surface 
tension against the adsorbent. Either Gibbs’ theory fails, which 
one is unwilling to conclude, or, at least, it offers nothing to the 
solution of the problem. 


236 COLLOIDAL BEHAVIOR 


DIFFICULTIES OF THE CONCEPT OF SURFACE TENSION AND 
ADSORPTION 


One reason why the parallelism of the surface tension against 
air and against a solid adsorbent is lost is doubtless the formation 
of electrical charges on the surfaces. Every electric charge 
must affect the surface tension. We may divide the total 
tension into two components: the purely mechanical tension, 
which causes the usual adhesion and cohesion in the absence of 
free electrical charges, and an electrically negative tension or 
expansion effect, due to the covering of the surface with electric 
charges of like sign. ‘This is shown by the fact that the pressure 
within a soap bubble, as measured by a manometer, diminishes 
when it is electrified. In Gibbs’ formula o can only be the 
total tension. It is noteworthy that Gibbs’ theory becomes 
inapplicable when the substance is of a decided electronegative 
or electropositive character, while for indifferent materials such 
as charcoal it is moderately applicable. However, these rela- 
tions are not yet clear, since charcoal also has, in general, an 
electrical potential difference against the solution. 

For mechanical surface tension we may draw the following 
picture. A molecule of a liquid is attracted equally by the neigh- 
boring molecules on all sides, and is held, therefore, under an 
internal pressure. A molecule lying on the surface occupies a 
unique position because the resultant of all the molecular attrac- 
tions is not zero. This results in a surface tension. But we 
may also use molecular attraction as the starting point of our 
consideration of adsorption. ‘The qualitative content of Gibbs’ 
theorem would be: ‘‘A substance dissolved in water is adsorbed — 
at the surface of another substance when its adhesion to the 
second substance is greater than its cohesion for its own phase.” 
This proposition is only a self-evident paraphrase. If one 
expresses in this way the relation so frequently quoted 
between surface activity and adsorption, the whole inadequacy 
of it will be apparent. All that remains of this oft-cited relation 
can be summed up as follows: If the adsorbent is charcoal and the 
adsorbate an organic non-electrolyte (or a weak electrolyte), it is 
generally true, although not without exceptions, that the ability 
of a substance to be adsorbed is parallel to its surface activity 
against air. 





ADSORPTION IN COLLOID SYSTEMS 237 


It is worthy of note that useful conceptions of adsorption have 
not resulted from the thermodynamic proposition of Gibbs, 
but have resulted by taking as a basis the force of molecular 
attraction. Polanyi’s theory of adsorption and also the well- 
known investigations of Harkins and Langmuir have so arisen, 
but cannot be discussed at this place. 

Molecular attraction of every degree exists between that of 
distinct chemical combination and that of the loosest adhesion. 
Formerly we distinguished sharply between chemical and phys- 
ical combination. Today it is no longer expedient to lay great 
stress upon this difference or to differentiate the adsorption 
phenomenon from a chemical reaction. Otherwise we will be 
faced with the embarrassment of being compelled to designate as 
adsorption the attraction of methylene blue by charcoal, and as no 
adsorption the attraction of methylene blue by kaolin. Further- 
more, in the case of charcoal we will be completely at a loss to 
establish a chemical equation for the process, while with kaolin a 
true substitution of calcium and similar ions* for methylene blue 
ions has been proved. This is surely a reaction which is generally 
accepted as chemical. At the same time it is not permissible to 
exclude the combination of methylene blue and kaolin from the 
group of processes known as adsorption. Numerous investiga- 
tions are described in the literature where an attempt is made to 
demonstrate a difference between chemical combination and 
adsorption. Thus, Bayliss® describes the following experiment. 
If we shake the colloidal blue solution of Congo red acid with 
aluminum hydroxide, there results immediately a blue adsorption 
compound, and only gradually is the red aluminum salt of the 
Congo red acid produced. 

It may be remarked that intermediate steps occur also with 
other chemical reactions, in which no doubt is entertained of the 
chemical character of the first step. Haller’ writes that the 
precipitates which result upon mixing basic and acid dyes of 
high molecular weight do not show the stoichiometric composition 
which would be expected of a salt of an acid and a basic dye, 
and that the composition of the precipitate isvariable. However, 


* Although kaolin is an aluminum silicate, it appears that under these 
circumstances calcium, present as an impurity, is always displaced more than 
aluminum. 


238 COLLOIDAL BEHAVIOR 


since such dyes tend to form colloidal micelles it is, indeed, quite 
possible that the micelles of one dye combine with the molecules 
of the other only at the surface. Such a compound cannot, of 
course, have a constant composition. Willstaetter® points out 
that wool, an optically active compound, adsorbs racemic alka- 
loids, but without any preference for the optically active com- 
ponent, which, however, would be expected in the formation of a 
true salt. This indicates without doubt that the compound 
formed by adsorption is not similar to a salt which may be 
prepared in a crystalline condition. It apparently behaves as 
a loose preliminary of true salt formation, but that this does not 
result from a chemical attraction may not be proved by this 
experiment. It is assuredly true and worthy of note that, with 
reactions taking place only at the surface, just such loose com- 
pounds are preferred. 

The following conception of Haber?’ is most important. In a 
crystal the molecules are held together by the forces of valence 
and, in general, these are satisfied. But the molecules lying on 
the surface are not completely saturated and the residual valences 
of the surface represent the field of adsorption possibilities. With 
amorphous substances such residual valences may be exaggerated. 
This theory explains the case where reciprocal adsorption is not 
manifest. But with reciprocal adsorption the main valences are 
effective. All this emphasizes a gradual transition, and it is, 
therefore, not advantageous to regard an adsorption as a condi- 
tion essentially different from that of a chemical reaction. To 
us, adsorption should signify chemical combination if it takes 
place at the surface of a substance and limits itself to the surface. 
But even with this definition we meet with transition cases. The 
example of permutite® places us again in error. This amorphous 
insoluble silicate, according to Schulze,!° exchanges its total 
content of sodium for calcium, and vice versa. The active surface 
extends so far inward, perhaps by capillary fissures, that every 
molecule can enter the reaction. There is here no longer any 
difference between molecules on the surface and those in the 
interior, and the surface reaction becomes a complete chemical 
reaction. All compounds formed at the interface between the 
dispersed phase of a colloidal solution and the dispersion medium, 
either with or without reciprocal adsorption, must in any event 


ADSORPTION IN COLLOID SYSTEMS 239 


be attributed to adsorption phenomena, and this chapter on 
“Adsorption in Colloid Systems”’ could, on this ground, be 
equally well entitled “‘The Chemical Reactions of Colloidal 
Substances.” It is evident that such a chapter can present only 
general points, for the special part of this chapter is at 
least half of the entire theory of colloids. 


THE SIGNIFICANCE OF THE SURFACE-ACTIVE NON-ELECTROLYTES 
FOR THE COLLOID STATE 


The stability of a colloidal solution is determined at any given 
moment by the surface tension between the disperse phase and 
the dispersion medium. A continued rise of this tension must 
lead to coagulation. As one, in general, would expect that 
surface-active non-electrolytes, like alcohols or esters, should 
rapidly lower the surface tension, so also would it be expected 
that these substances should act as stabilizing agents upon 
colloidal solutions, e.g., that with their help it should be quite 
possible to bring other substances into colloidal aqueous solu- 
tion. With these materials it would be anticipated that this 
property should maintain even in extremely dilute solution, 
since one always attributes a much higher concentration to the 
surface layer. In reality, however, any action of these sub- 
stances on the colloidal condition is very slight, and where it is 
at all present coagulation is nearly always favored rather than 
stabilization. In order to explain at all this slight activity 
we must call to mind once more the fact, often little appreciated, 
that all of these so-called surface-active substances reduce the 
surface tension of water only against air, water vapor and char- 
coal, but not, in general, against all surfaces. 

The most surface-active substances, such as heptyl and octyl 
alcohols, fatty acid esters, and higher urethanes, are not in the 
slightest degree adsorbed by such substances as kaolin, alumina, 
ferric hydroxide, or, indeed, by most other inorganic materials, 
even though the adsorbent be brought into the finest state of 
subdivision. ‘Thus, a sol of ferric hydroxide with its particles 
the size of amicrons, in the highest possible concentration, adsorbs 
no trace of octyl alcohol, etc., as was shown by Michaelis and 


240 COLLOIDAL BEHAVIOR 


Rona® with the aid of a compensation dialysis. Adsorption was 
only in an extremely low degree demonstrable, even under most 
favorable conditions, with several other substances, such as 
tale, cellulose (filter paper), and sulfur. It was best with sulfur, 
while for such traces of adsorption as occurred with tale or 
filter paper there is present the objection that the slightest con- 
tamination of the surface with fats would vitiate the observations. 

Except in the presence of electrolytes, the non-electrolytes are 
generally without influence upon the condition of the colloid and 
it is, therefore, a matter of indifference whether the non-electro- 
lyte is surface-active or not. From numerous observations 
may be mentioned the finding of Freundlich" that methyl and 
ethyl alcohols, urea, cane sugar, and phenol, even at high con- 
centrations, are without action upon arsenic sulfide sol, and, 
according to Wo. Ostwald,'!? the same is true of the very dis- 
similar Congo red sol. With non-electrolytes which are miscible 
with water in all proportions, as ethyl alcohol, it must be remem- 
bered that at high concentrations it is no longer an aqueous 
solution, and the fact that finally an aqueous sol can be precipi- 
tated by ethyl alcohol is certainly not a matter of concern in a 
chapter on adsorption. Moreover, the strong surface-active 
non-electrolytes are all so difficultly soluble that a like condition 
does not then come into question. The essential property which 
concerns us in this phenomenon is, indeed, the dielectric constant 
of the solvent, and this is but little altered by strong capillary- 
active substances because of the too slight solubility, although 
in the pure state their dielectric constants are much smaller 
than that of water. 

The situation is different when such water-insoluble substances 
as the higher alcohols, benzol, or oils are shaken in undissolved 
condition with any substance in colloidal solution. When the 
emulsion of this oil produced by shaking separates, it often carries 
down the colloid also. This is as true for the protein type 
of solutions as for the suspension colloids. Here the dissolved 
colloidal material acts not as an adsorbate but as an adsorbent. 
The phenomenon was investigated by Zsigmondy?* for gold 
sol. This is, in general, coagulated by shaking with benzine, 
carbon disulfide, etc., the gold separating as a shiny membrane 
at the interface of the liquids. The phenomenon does not persist 


ADSORPTION IN COLLOID SYSTEMS 241 


when the gold particles are very small. However, these cases do 
not belong here. 

In the presence also of electrolytes, surface-active substances 
in the dissolved state often show a sensitizing action upon a 
colloid. The quantity of electrolyte just necessary for coagula- 
tion is decreased through its presence. Freundlich and Rona™ 
found that the coagulating concentration of sodium chloride 
-upon a ferric hydroxide sol was reduced, by the addition of 10 
millimols of camphor per liter, from 35 to 27 millimols of 
sodium chloride per liter or, by the addition of 5 millimols 
of thymol, to 20 millimols. The action of these substances 
increases rapidly in homologous series, as does the surface tension, 
according to the rule of Traube. Closely related to this is the 
observation of Rona and Gydorgyi” that the settling of kaolin 
suspensions is accelerated by the addition of camphor, thymol, 
or tributyrin. Itis extraordinary that this action is not observed 
in the settling of charcoal suspensions. Thus, there is no action 
with those powders which adsorb these substances while there is 
an action with those which do not adsorb them. ‘There seems 
to the writer to be no satisfactory explanation yet advanced for 
this phenomenon. Freundlich thinks that the change in the 
dielectric constant of the water is the essential factor. But, in 
order to admit a change in dielectric constant, one must also 
assume that these substances become more concentrated on the 
surface, for, in the concentration in which they usually exist in 
aqueous solution, these constants do not materially change. 

Experience shows, however, that these substances do not 
become concentrated on the surfaces toward which they are 
active. One might, therefore, believe that the surface-active 
substances influence the adsorption of the electrolyte in some 
way, and, through this, change the potential of the surface. 
But in such cases as are at all approachable by chemical analysis, 
Lachs and Michaelis®® showed that an opposing influence of 
electrolyte and non-electrolyte as regards adsorption does not 
take place at all, and the divergences from this principle which 
have been found are quite insignificant and apparently limited to 
special conditions. One case in point may be found desirable for 
further investigation. According to Freundlich and Rona, 
sensitization takes place only with monovalent, difficultly adsorbed 


242 COLLOIDAL BEHAVIOR 


ions, and not with divalent and better adsorbed ions. ‘Thus, 
camphor sensitizes the action of sodium chloride, but not that of 
calcium chloride upon ferric hydroxide sol. ‘They explain this 
as follows:4@: 8) Coagulation begins as soon as a determined 
amount of cations is adsorbed, and this minimum amount is 
reduced by the addition of non-electrolytes. When, now, the 
quantity of adsorbed ions is reduced to a certain value, so also 
must the concentration containing an equal amount of ions in 
solution be reduced to a determined value. With the mono- 
valent ions which are little adsorbed and have a flat adsorption 
curve, this amount is noticeably large. On the other hand, with 
the bivalent ions showing a steeper adsorption curve, it is very 
small. 

Entirely unexplained, however, is the observation of Kruyt 
and van Duin" that the coagulation of arsenic sulfide sols by 
amyl alcohol, phenol, etc. is sensitized by the addition of mono 
and trivalent ions but, on the other hand, is weakened by the 
addition of di and tetravalent ions. 


Tur ADSORPTION OF ELECTROLYTES 


In contrast to the little-investigated and slight action of the 
non-electrolytes on colloids is the well-known behavior of electro- 
lytes. The addition of an electrolyte is the method most gener- 
ally used for intentionally altering the nature of a colloid. ‘This 
action is intimately concerned with adsorption, for all the 
surfaces, which behave indifferently towards non-electrolytes, 
have the ability to adsorb electrolytes in large measure. ‘This 
adsorption of the electrolyte takes place in a manner which is depen- 
dent on the nature of the adsorbent, z.e., whether it is of the char- 
coal type, which is incapable of ion formation, or it has the consti- 
tution of an ionogen, capable of dissociation into electropositive 
and electronegative constituents, such, for example, as silicic 
acid (exhibiting a tendency to dissociate into silicate ion and 
hydrogen ion) or a metallic hydroxide or some other similarly 
constituted substance. If an electrolyte such as sodium chloride 
or methylene blue chloride is brought in contact with charcoal, 
there is no other possibility but that the positive and negative 
ions shall be adsorbed in equivalent amounts. This adsorption 


ADSORPTION IN COLLOID SYSTEMS 243 


can be designated as equivalent adsorption of electrolytes. The 
above prediction was borne out by analyses performed by 
Rona and the author.'”.'!8 The anion and the cation were 
always adsorbed in equivalent amounts. This equivalence 
was not exactly obtained, but the error of from 5 to 20 per 
cent may easily be shown to be due to impurities on the surface 
of the charcoal. For example, with methylene blue it was 
found that, in the filtered solution, after complete exhaustion 
of the methylene blue, some chlorine ion still was present, and 
this was shown to exist as calcium chloride. The calcium was 
evidently obtained from the charcoal, and indicates the existence 
on the surface of a lime salt, possibly a silicate, that has been 
released according to the principle of reciprocal adsorption. 
Except for this slight variation, the adsorption of the methylene 
blue base and the chlorine were equivalent. 

It is also conceivable, theoretically, that an hydrolysis accom- 
panies the adsorption, according to the reaction: 
Charcoal+methylene blue chloride—Charcoal-methylene blue base + HCl 
Although such a reaction has formerly been postulated, there 
has been no experimental proof of its existence. It has been 
asserted that when charcoal is placed in a basic dye (the salt of 
a color base with a mineral acid), and the color removed, an acid 
solution remains, while, if an acid dye is used, an alkaline solution 
results. This, however, is not the case. If a salt to be adsorbed 
is already noticeably hydrolyzed, as perhaps aniline hydrochloride, 
the case is different. But, for all salts which are not noticeably - 
hydrolyzed, it is possible to state the general fact that hydrolysis 
is not induced by the process of adsorption. This applies not 
alone to charcoal but to other absorbents of that type. A few 
exceptions will be pointed out later. 

Although the relatively crude method of chemical analysis 
seems to show that the adsorption of anions and cations takes 
place in equivalent amounts, it is still possible that this adsorp- 
tion deviates from exact equivalence to a degree that is imper- 
ceptible by such methods, in that a slightly greater amount of 
the cation than of the anion may be absorbed, or vice versa. 
This phenomenon apparently causes an electrical potential 
at the surface, wherein an electrical double layer is formed, the 
layer adjacent to the charcoal consisting of the more strongly 


244 COLLOIDAL BEHAVIOR 


adsorbed ions, and the layer further removed from the charcoal, 
of the more weakly adsorbed ions. There would appear an 
electrical potential difference between the charcoal and the 
water which could be revealed by cataphoresis of the charcoal 
particles under the influence of an external electric field or, 
under other experimental conditions, by electro-endosmose. 
The charge of the colloid particles may most probably be attrib- 
uted exclusively to this adsorption potential, and all experi- 
ments up to the present have given facts in confirmation of 
this hypothesis. It can be demonstrated’, 7° that, in all cases, 
if a sufficiently adequate experimental method is employed, 
blood charcoal is positively charged when the cation is most 
strongly adsorbed, and negatively charged when the anion is 
most strongly adsorbed. 

The following evidence in favor of this theory is deduced from 
experimental data. From analytical experiments with strong 
acids and bases it was demonstrated*° that H+ and OH7 ions are 
about equally well adsorbed, and that both of these ions are more 
readily adsorbed than most other ions. In confirmation of 
this may be cited experiments on endosmose.*! 1% 2° Blood 
charcoal is charged positively by acids and negatively by bases, 
with an isoelectric point at pH 3.0. Sugar charcoal, on the 
other hand, does not adsorb the acid anions at all, not even the 
very easily adsorbed anions of the acid organic dyes. In agree- 
ment with this we find that sugar charcoal is never charged 
. positively by acids, but always negatively. This negative charge 
may, indeed, be diminished by the use of strong acids, but never 
reversed. On the other hand, among the strong acids there was 
only a single one found which was not capable of imparting a 
positive charge to blood charcoal, namely, sulfo-salicylic acid. 
Now it was found that sulfo-salicylic acid is absorbed by char- 
coal much more strongly than is any inorganic acid. We are 
forced to the conclusion that the anion of this acid is more 
actively adsorbed than the anion of a mineral acid, more actively 
even than the hydrogen ion. In this case, the portion of the 
double layer adjacent to the surface of the charcoal must always 
be negative in the presence of sulfo-salicylic acid. This agrees 
with the data obtained in experiments on endosmose. ‘The 
theory postulating the charging of charcoal through inequivalent 


~ 


ADSORPTION IN COLLOID SYSTEMS 245 


ion adsorption seems herewith established. The theory is, 
however, not applicable to the case of the stability of charcoal 
suspensions, since these contain a protective colloid, as is the 
case in india ink. The surface is then not charcoal, but a film 
of the protective colloid. 

The case of an ionic adsorbent is of greater interest from the 
colloid point of view. We will consider first the case of recipro- 
cal adsorption, as this has been experimentally established and is 
of general occurrence. The adsorption proceeds as would be 
anticipated from a purely chemical point of view. For example, 

Ca silicate + methylene blue chloride—Methylene blue silicate + CaCl. 
whereby the CaCl, is found in the solution. 

Basic ferric chloride (e.g., colloidal ferric hydroxide which contains Cl- ion) 
+ Na eosine—ferric eosinate + NaCl, 

whereby the NaCl is found in the solution. In like manner, 
in the reaction of a free amorphous acid with an adsorbable 
salt, as, for example, 

Silicic acid + methylene blue chloride— Methylene blue silicate + HCl, 
adsorption can only be possible if hydrolysis of the salt sponta- 
neously occurs. 

Such reactions, however, have not been realized experimentally 
up to the present time.'® For one thing, it is exceedingly difficult 
to prepare such amorphous bases or acids completely free from 
their salts. The purest silicic acid that has been prepared always 
contains calcium, and, when it is colored with methylene blue 
chloride, it is found to take up only an amount of methylene 
blue which is equivalent to the calcium which it gives to the 
solution. The same can be shown equally well with cellulose 
(filter paper). The purest metallic oxides which adsorb at all 
always contain anions in combination, as, for example, the com- 
mon ferric hydroxide always contains chlorine, and when it is 
colored with eosine the reciprocal exchange of the anions always 
takes place. If nickel oxide is prepared by heating nickel nitrate, 
the oxide will adsorb eosine only so long as nitrate or nitrite ions 
are replaced. If these are completely expelled beforehand, the 
oxide will not adsorb at all. Nickel oxide was selected because, 
after heating, it can be rubbed to a fine powder to which we can 
attribute a large surface sufficient for active adsorption. ‘The 
objection that the heating may have affected the surface unfavor- 


246 COLLOIDAL BEHAVIOR 


ably cannot entirely be refuted, but it is worthy of mention that a 
metallic oxide has not yet been prepared which adsorbed an acid 
dye except by ion exchange, it being presupposed that the dye- 
stuff does not already possess marked colloidal properties. 

However, the possibility of hydrolysis as a result of adsorp- 
tion cannot be generally denied. Neither is it apparent why, 
under suitable conditions, H+ ions should not be displaced 
from the surface by metallic ions. Van Bemmelen*! long ago 
described a striking example of this kind. When freshly pre- 
cipitated and washed manganese dioxide is placed in the solu- 
tion of a neutral salt, such as sodium chloride or sodium sulfate, 
the sodium is adsorbed in exchange with hydrogen ions, that is, 
a small quantity of free mineral acid is formed in the solution. 
The writer has obtained from such an experiment an HCI solution 
of pH 4. It has further been shown by Linder and Picton, 
as also by Whitney and Ober,?’ that, in the precipitation of an 
arsenic sulfide sol by metallic salts, the metallic cation was 
exchanged with the H+ ion, that is, the acid was found free in the 
filtrate. It may be assumed that the same would result by the 
coagulation of mastic with neutral salts. In general, therefore, 
we may not expect hydrolysis by adsorption with a substance as 
electroneutral as charcoal, but there is no reason to deny the possi- 
bility of such a reaction with colloids of a decided acid or basic 
character. It is, indeed, extraordinary that the anticipated 
hydrolysis in many cases fails to become manifest. This need 
not be interpreted as a real absence of a chemical affinity, 
but as a kind of passivity. 

An observation in agreement with our point of view is that in 
each case where a reciprocal adsorption does take place it can 
be interpreted adequately on the basis of an anticipated chemical 
exchange. Adsorption occurs when a more difficultly soluble 
substance will result thereby. Thus, in the special case of 
manganese dioxide it may be assumed that an insoluble sodium 
salt of manganous acid is produced, otherwise obtained only 
through the fusion of the components. 

In good agreement with such a chemical conception is the 
observation of Fajans and Beer?’ and also of Paneth and Horo- 
vitz** that a radioactive element of a difficultly soluble and hetero- 
polar adsorbent is strongly adsorbed when it forms-a difficultly 


ADSORPTION IN COLLOID SYSTEMS 247 


soluble compound with the same, it being previously ascertained 
to be present in excess. The chemistry of adsorption is not 
simple, for the chemical surface of the colloids is not exactly 
known. Since, for example, according to Freundlich, the sur- 
faces of the particles in a sulfur sol prepared by the method of 
Odén consist of a polythionic acid, so may we also expect that 
other simple or even elementary colloids should exhibit a complex 
condition at their surfaces. 

Still another type of adsorption is that known as electrolytic 
adsorption, which does not depend on an exchange, but simply on 
attraction of the ions of a solid surface which binds them to the 
surface of the adsorbent. The best known case is that of the 
silver halides. When a halogen salt, as sodium iodide, is added to 
a solution of silver nitrate, a precipitate of silver iodide results. 
Lottermoser”* pointed out that the electric charge on the precipi- 
tated silver iodide is dependent on the quantity ratio of the two 
salts. If the precipitation is effected with an excess of the halo- 
gen salt, the precipitate is electronegative; with an excess of the 
silver nitrate it is electropositive. The evident explanation is 
that the surface of the silver halide is covered in the one case 
with an excess of iodide ion, and in the other case with an excess 
of silver ion. Lottermoser and Rothe” were actually able to 
demonstrate the adsorption, whether it was produced by the iodide 
or the silver nitrate. Other properties of the silver iodide also 
were affected by the nature of the charge. The positive residue 
is the more sensitive to light; the negative, according to Fajans 
and Beckerath,?® easily adsorbs the positive ions of the radio- 
active isotope of lead, while the positive residue does not. This 
adsorption capacity of the surface of the silver iodide for one of the 
two kinds of ions of which it is itself composed is best explained by 
the theory of Haber.?”:*8 Each silver ion lying on the surface of a 
crystal of silver iodide (assuming the principle of a cubic space- 
lattice of silver and iodide ions) has a residual valence for one 
iodide ion, and each iodide ion lying on the surface has a residual 
valence for one silver ion. Depending on the quantity ratio of 
the ions in the solution, either one or the other is adsorbed in 
excess, and this determines the charge on the surface. In con- 
trast to the reciprocal adsorption (Michaelis and Rona!® and 


248 COLLOIDAL BEHAVIOR 


Paneth”’), the above type may be designated as contact adsorp- 
tion, as suggested by Fajans.*° 


EXPERIMENTAL PROOF OF THE RELATION BETWEEN Ion ADSORP- 
TION AND THE COAGULATION OF COLLOIDS 


Numerous instances have been brought forward in proof that 
an electrolyte which produces the coagulation of a colloid is itself 
adsorbed during the precipitation. In all cases of the following 
types that have been investigated, this above rule has been found 
to be the case: when the coagulated ion is adsorbed rather easily 
by the colloid of opposite charge, when it is a polyvalent ion, 
when it is a monovalent ion of high adsorbability as the silver ion, 
or when it is an organic dye or alkaloid ion. In fact, there are 
only two groups of cases known where an adsorption of the 
electrolyte has not been demonstrated. The first group embraces 
cases such as the coagulation of a colloidal acid, for example 
mastic, by an acid in solution, such as hydrochloric or acetic, 
the solution becoming in no wise impoverished in acid content. 
The second case can occur with negative colloids when the 
coagulating ion is a monovalent and difficultly adsorbed ion, 
such as sodium. 

Cases may first be cited in which the demonstration of adsorp- 
tion has been readily achieved. The simplest of these, which 
affords an excellent demonstration of the relation between 
coagulation and adsorption, is found in those cases where the 
precipitate contains a colored ion. For example, when mastic 
is precipitated by a basie dye, such as fuchsin, the coagulum is 
colored red. Such examples are numerous. Linder and Picton?? 
have shown that the cation of the electrolyte used in the coagula- 
tion of arsenic sulfide sol is adsorbed, indeed with an exchange of 
H* ion. It was found by Whitney and Ober?* and by Pauli and 
Matula*! that, in the coagulation of ferric hydroxide sol by a 
sulfate, the sulfate ion is adsorbed in exchange with the chloride 
ion of the micelles. Michaelis, Rona, and Pincussohn*? showed 
that, in the coagulation of a mastic sol, the cation of the coagulat- 
ing electrolyte is always adsorbed, but such proof could not be 
obtained with the cations of the monovalent alkali metals, and, 
furthermore, adsorption could not be demonstrated on coagula- 
tion with acids. 





ADSORPTION IN COLLOID SYSTEMS 249 


The difficulty lay in the fact that with salts so inactive and 
diffiicultly adsorbed as sodium chloride the concentrations neces- 
sary to bring about coagulation are very much greater than 
is necessary with active and easily adsorbed ions. Thus, in 
order to cover the surface of the mastic with a given quantity 
of ion equivalents, the adsorbed salt, in the case of sodium chlo- 
ride, must be in equilibrium with a much greater concentration of 
salt in solution than would be the case with calcium chloride. 
The relative per cent adsorption of the sodium chloride in a given ° 
coagulum will, therefore, be much smaller than in the case of 
calcium chloride. The actual loss of salts in the solution, even 
of the easily adsorbed salts, is only a few per cent of the total 
quantity, so it is to be expected that, with sodium chloride, the 
loss amounts to only a few tenths of 1 per cent or less. A loss 
of this order cannot be proved by chemical analysis. There 
is no compelling reason, however, for assuming any divergence 
in this case. 

The coagulation of colloidal acids by acids in solution is a 
different matter. With mastic, hydrogen ions are the most 
active of all of the common ions. Even at a concentration of 
10-4 nN, coagulation proceeds rapidly. It would be expected, 
therefore, that, if adsorption takes place at all, it would 
be very easily discovered. But an actual adsorption of an 
acid cannot be demonstrated. Nor can the competence of 
the analytical method here be questioned. We are, therefore, 
forced to the hypothesis that the coagulating action of the acid is 
not accompanied by an adsorption of the acid. Any compre- 
hensive theory concerning the relation between adsorption and 
coagulation must account for this special case. 

Even in those instances where an electrolyte is added to a sol in 
amounts too small to effect coagulation, adsorption may usually be 
demonstrated by ultrafiltration and analysis of the ultrafiltrate, or 
by a potentiometric ion analysis of thesolution. Thesame quanti- 
tative relations as are expressed by the so-called adsorption iso- 
therm of Freundlich may be found by a variation of the quantity 
of electrolyte added as wellas by the usual adsorption experiments. 
Kohlschiitter*? demonstrated the adsorption of the electrolyte 
by silver sol, and Lottermoser and Mafhia**: 44 by ferric hydroxide 
sols. The latter investigators, and also Pauli and Matula,*! 


250 COLLOIDAL BEHAVIOR 


made use of the principle of potentiometric measurements of ion 
concentrations in the solutions. The experiments of Lottermoser 
on adsorption, either of silver ion or of halogen ion, by silver 
halides have already been discussed in an earlier section. 


THEORY OF THE MuTUAL RELATION BETWEEN ION ADSORPTION, 
Ionic DISCHARGE, AND COAGULATION 


The theory relating ion adsorption and the coagulation of 
colloids takes for its basis the following postulates: 

1. Every electrical charge on the colloid particles favors 
their stability; discharge favors their coagulation. 

2. Adsorption leads to a discharge or, at least, a partial dis- 
charge and results, therefore, in a flocculation. We are now 
concerned with the proof that the coagulation of the colloid is 
accompanied not only by an adsorption of the electrolyte, which 
nearly always occurs, but also by the discharge of the particles, 
which is always effected. This proof is obtainable, in general, 
by the method of electrical cataphoresis. The velocity of ca- 
taphoresis of a particle surrounded by an electrical double layer 
depends, according to Helmholtz, on the potential of the double 
layer: 

y _SKH 


Airy 


where V is the velocity of cataphoresis, ¢ the potential of the © 
double layer, K the dielectric constant of the solvent, 47 anumeri- 
cal factor, and 7 the absolute viscosity of the solvent. The 
factor K was neglected by Helmholtz and was first introduced 
by Pellat and Perrin,** on the assumption that the space between 
the two shells of the double layer could be regarded as possessing 
the same dielectric value as the external solvent. The view was 
formerly suggested in communications by Hardy that coagulation 
resulted only upon the complete discharge of the double layer. 
It is not surprising, however, that later investigations showed 
that complete discharge was not necessary; that a diminishing 
of the charge to a certain low value was sufficient to effect the coag- 
ulation. This begins always, for any given sol, when, no matter 
how this value is achieved, the potential has fallen to a definite 





ADSORPTION IN COLLOID SYSTEMS 251 


absolute value. This was first pointed out by Ellis*4 and Powis*> 
for oil emulsions. It was immaterial whether the discharge was 
effected by ThCli, AlCl;, BaCls, or KCl. The coagulation of the 
emulsion began in each case when the potential had decreased to 
30 to 40 millivolts (Th 40, Al 30, Ba 28, K 30 millivolts). This 
represents, therefore, the critical potential. It will be observed 
that the agreement of the critical potentials for different cations 
is not perfect, and even greater differences were observed in the 
coagulation of arsenic sulfide sol, investigated by Powis. These 
were as follows: Th(NOs)4 26, AICI; 25, BaCl, 26, but KCl 44 
and HCl 30 millivolts. Even here, however, the values are of the 
same order, and the conclusion is reached that the magnitude of 
the potential of the double layer is, indeed, the most important, 
if not the only decisive, factor in the coagulation. 

Both concepts referred to at the beginning of the chapter remain 
correct as stated, but now require a more detailed discussion. 

Although the content of both of these postulates is today 
entirely familiar to colloid chemists, their inner significance is 
very often not clearly understood. The relation is frequently 
stated in the following manner: When a negative particle adsorbs 
positive ions it must be discharged, and, therefore, coagulating 
action is associated only with ions of opposite charge and the 
ability to adsorb is proportional to the magnitude of the differ- 
ence in charge. ‘This description appears at first sight to be 
very simple, but it is, when so expressed, not strictly repre- 
sentative of the state of affairs. It implies the somewhat naive 
concept that the colloid particles are charged in a manner similar 
to that of a stick of sealing wax which has been rubbed, and 
that the discharge through adsorption of an oppositely charged 
ion is analogous to discharge of the negative stick of sealing wax 
by covering with a positively charged plate of tin foil. The 
difference, however, lies in the fact that the colloid particles are 
not charged in the same manner as the sealing wax, but rather 
by virtue of being surrounded with an electric double layer. 
The analogy would be to a stick of sealing wax closely surrounded 
by the cloth, through the rubbing of which the electricity is 
produced. As soon, however, as we remove the cloth we disrupt 
the ‘‘double layer.”’ The electricity becomes ‘free’? and we 
have then no analogy with colloids. It is further assumed that 


202 COLLOIDAL BEHAVIOR 


colloid particles of like charge cause an electric repulsion and so 
lessen the probability of collision of the colloid particles. But 
when we have a perfect double layer, this repulsion is not under- 
standable on the basis of Coulomb’s law without further ampli- 
fication. It we imagine the colloid particles as consisting of 
spheres, we can picture the outer energy effects of the electricity 
of the surface-charged particles as if the total charge were con- 
centrated at the center of the sphere. Since the inner and 
outer shells of the double layer have the same charge of opposite 
sign and have a common middle point, they immediately neu- 
tralize each other, and an energy component extended outwards is 
not explained. The solution of this paradox lies in the following. 
We may regard the charge on the surface of a sphere as concen- 
trated at its center only when the magnitude of the charge is the 
same over the entire surface, and this will be the case with a 
given colloid particle. But if two such particles approach very 
close to each other, a change in the distribution of the charge will 
take place, due to the electrostatic action. The moving outer 
double layers becomes orientated, because of the electrostatic 
repulsion, so that the double layer is thinner on the sides turned 
towards each other and denser on the sides turned away from each 
other. We may, therefore, no longer regard the charge as 
concentrated in the center of the sphere but rather in the center of 
an electrostatic field, which is not coincident with the geometric 
center. This idea may be developed further and leads to the 
conclusion that the spheres repel each other, but not by the simple 
postulate of Coulomb’s law, inversely proportional to the square 
of the distance, the repulsion decreasing much more rapidly with 
increase of distance. The repulsion is, therefore, imperceptible 
when the particles are far apart, and becomes apparent only when 
the particles approach very close to each other. Nor does the 
double layer itself act to prevent contact by exerting a repulsive 
action from a distance. Indeed, the behavior may be likened 
to that of an elastic cushion, which at first offers no opposition to 
approach, but at the moment of contact exerts a repulsive power. 

The relation of adsorption to ionic discharge now remains to 
be discussed. ‘The mechanism of the discharge through ion 
adsorption is divided for consideration into contact adsorption 
and reciprocal adsorption. 





ADSORPTION IN COLLOID SYSTEMS 253 


With contact adsorption the case is a simple one. Let us 
return to the example of silver iodide. Fajans and Franken- 
burger®® have again called attention to a phenomenon that may 
easily be carried out by any analyst. If a titration is undertaken 
wherein AgNO; is added gradually to a solution of Nal, there is 
first formed a cloudiness of the AgI, which has little tendency to 
settle, and a colloidal solution results. As we approach the end 
point the precipitate rapidly coagulates and separates. If we 
pass beyond the end point, however, by rapid overtitration, 
the precipitate remains as before with no decided tendency to 
coagulate. The conditions for flocculation are dependent on the 
discharge. Before the end point is reached the particles are 
negative through adsorption of iodide ions, but upon overtitra- 
tion the particles are positive through adsorption of silver ions. 
Here there is no difficulty in the interpretation. 

With reciprocal adsorption the case is not so simple. In this 
type we may think of the charge as arising in the following 
manner: The substance of each colloid particle exerts a tendency 
to dissociate into ions, one of these being of the usual type and 
the other, being incapable of diffusion, remaining associated as 
a micelle. Thus a double layer results, and the laws of the 
electrical condenser may be applied, with slight modification, 
as has been done by Helmholtz. 

Accordingly, in the double layer, we may differentiate the 
following factors: 

1. The electrical surface density o of each layer. 

2. The distance between the two layers or the thickness D 
of the double layer. 

3. The potential difference p of the layers, or, to be more 
exact, the difference in the potential which the total charge of 
the condenser exerts upon a point of one layer and that 
which it exerts upon a point in the other layer. To relate these 
factors: 

_ 4roD 
sues 
where K is the dielectric constant of the medium between the 


layers. But Gouy* has pointed out that at least the outer shell 
of the double layer, which consists of the free-moving ions, cannot 


254 COLLOIDAL BEHAVIOR 


be conceived as a surface layer, but possesses a certain depth or 
field of diffusion, in that the concentration of the ions constitut- 
ing the outer shell gradually becomes identical with the con- 
centration of the same kind of ions in the solution. 

We may not, therefore, speak of a definite thickness of the 
double layer, but may picture the action only as similar to that 
of a surface condenser, in which the thickness is equal to that 
of a certain average thickness of the double layer of the diffuse 
condenser. ‘This thickness is, indeed, only a calculated value, and, 
if we wish to make use of it, we may simplify the theory in the 
following manner. Since the surface of a given colloid particle 
has a definite number of molecules capable of dissociating, the 
density of the charge on a given colloid will not, in general, be 
variable. All diffusible ions will remain at a definite distance 
from the colloid ion, depending on the nature of the diffusible ion. 
In determining the potential of such a condenser, only the 
average thickness of the double layer need be considered. The 
dielectric constant also should be considered, for we cannot 
assume that the medium between two nearly adjacent ions is 
the same as that between two distant ions, which is simply the 
solvent. In all cases we may conceive the condenser to be 
changed only by varying the average thickness, if we wish to keep 
clear the concept of a fixed potential. The value of the concept 
lies in this, that upon the magnitude of this potential depends 
the mutual repulsion of the micelles. If all the ions of the outer 
shell are placed infinitely near to the oppositely charged ions 
of the inner shell, the potential of the condenser is zero, and 
two such condensers taken as a whole exert no influence upon 
each other. 

By combining this view with the experience that, in general, the 
coagulating ions undergo reciprocal adsorption with others, we 
arrive at the following generalization. The adsorption of an 
ion signifies the replacement of an ion, present originally in the 
outer shell of the double layer, by another of like charge. This 
exchange takes place when the new ion is able to approach 
closer to the inner shell of the double layer and hence to diminish 
the potential difference between the two layers and hence to 
diminish the repulsion of the micelles. Adsorption takes place 
when, by its accomplishment, the potential electrial energy of 





ADSORPTION IN COLLOID SYSTEMS 255 


the colloid particles will be diminished, or when electrical work 
may be done by the adsorption. 

We are not able to say, in general, what property, according to 
this view, an ion must have in order to displace another ion from 
the outer shell of the double layer. But several general observa- 
tions are at once clear. A bivalent ion always tends to displace 
a univalent ion, as the former can always approach closer to the 
inner shell of the double layer than the latter. The electrical 
attraction of the inner shell upon two univalent ions and upon 
one bivalent ion is the same, but, other conditions being the 
same, the diffusion pressure opposing this attraction is twice 
as great for two univalent ions as for one divalent ion. Still 
other constitutional influences may play a part which cannot 
here be surveyed individually. The strongest influence is 
that of valence, as can be easily understood. 

It still remains for us to bring into harmony with this concept 
the fact that, for example, mastic is coagulated by acid without 
the latter being adsorbed. The outer shell of the double layer 
consists, no doubt in the case of mastic, of H+ ions only, when 
the colloid is in pure water. ‘These H* ions are dissociated from 
the mastic, not adsorbed from the water. The average thick- 
ness of the double layer is determined, on the one hand, by 
the electrostatic attraction of both layers for each other; on 
the other hand, by the diffusion gradient of the Ht ion from 
the outer shell of the double layer to the rest of the solution. 
This gradient depends on the concentration of the Ht ion in 
the solution. An increase in concentration causes a decrease 
in the average thickness of the double layer and leads to a 
decrease in potential, and, therefore, also to coagulation even 
without adsorption. 

The foregoing may be summarized in the following proposition. 
A colloid is discharged and coagulated by an electrolyte when, 
either through the exchange of one kind of ion by another kind of 
ion or through opposed osmotic action of the same kind of ion 
which forms the outer shell of the double layer, the potential 
electrical energy of the system can be diminished. A quantitative 
formulation of this principle appears, however, as yet premature, 
since too many necessary data are missing. 


250 5 COLLOIDAL BEHAVIOR 


REFERENCES 


. Grpss, J. WituarD: Trans. Connecticut Academy, 2 and 3 (1875-1878). 
. Tuomson, J. J.: ‘“‘Applications of Thermodynamics to Physics and 


Chemistry.” 


. Micuagris: ‘Die Wasserstofhonenconcentration,”’ 2nd ed., Berlin, 


1922, p. 200. 


. Freunpuicu: ‘‘Capillarchemie,”’ 2nd ed., Leipzig, 1922. 

. Micuaruis and Rona: Biochem. Z., 102 (1920), 268. 

. Baytiss: Proc. Roy. Soc. (London), 84 (1914), 586. 

. Hatter: Kolloid-Z., 22 (1918), 113; 24 (1919), 56; 27 (1920), 30. 


WILLSTAETTER: Ber., 37 (1904), 3758. 


. Gans: Jahrb. kgl. Preuss. Geolog., 26 (1905), 179; 27 (1906), 63. 

. ScHULZE: Z. physik. Chem., 89 (1915), 168. 

. Freunpuicu: Jbid., 44 (1903), 136. 

. OstwaLD, Wo.: Kolloidchem. Bethefte, 10 (1919), 204. 

. ZstaMonvy: Z. Electrochem., 22 (1916), 102; Z. anorg. allgem. Chem., 96 


(1916), 265. 


. FrRevuNDLICH and Rona: Biochem. Z., 81 (1917), 87. 

. Rona and Gr6reyt: Jbid., 105 (1920), 133. 

. Kruyt and van Duin: Kolloidchem. Beithefte, 5 (1914), 269. 

. Rona and Micuaruis: Biochem. Z., 94 (1919), 240. 

. Micwarxis and Rona: Jbid., 97 (1919), 57. 

. Micuaeuis: Z. Electrochem., 28 (1922), 453. 

. Umetsu: Biochem. Z., 135 (1923), 442. 

. VAN BEMMELEN: J. prakt. Chem., 23 (1881), 342; ‘‘Die Adsorption,” 


Dresden, 1910. 


. LinDER and Picton: J. Chem. Soc., 87 (1905), 1908. 
. Wuitney and OBER: Z. physik. Chem., 39 (1902), 630. 
. LorrermoseEr: J. prakt. Chem., 72 (1905), 39; 73 (1906), 374; Z. physik. 


Chem., 60 (1907), 451. 


. LoTTERMOSER and RotueE: Z. physik. Chem., 62 (1908), 359. 
. Fasans and Breckeratu: [bid., 97 (1921), 478. 
. Haper: J, Soc. Chem. Ind., 33 (1914), 50; Z. Electrochem., 20 (1914), 


o21, 


. Lanemuir: Chem. Met. E'ng., 15 (1916), 469; Phys. Rev., 6 (1915), 79; 


8 (1916), 149; J. Am. Chem. Soc., 38 (1916), 2221; 39 (1917), 1848; 40 
(1918), 1361. 


. PAnEtTH: Z. physik. Chem., 101 (1922), 445. 

. Fasans and FRANKENBURGER: Ibid., 105 (1923), 255. 

. Pauxti and Maruua: Kolloid-Z., 21 (1917), 49. 

. Micuar.is, Pincussonn, and Rona: Biochem. Z., 6 (1907), 1. 
. Perrin: Chim. Phys., 2 (1904), 601; 3 (1905), 50. 

. Exuis: Z. physik. Chem., 80 (1912), 597. 

. Powis: [bid., 89 (1915), 186. 

. Harpy: Proc. Roy. Soc. (London), 66 (1900), 110. 





37. 


38. 


39. 
40. 
41. 
42. 
43. 
44, 
45. 


ADSORPTION IN COLLOID SYSTEMS 257 


Fasans and Brrr: Ber., 46 (1913), 3486; Fasans and Ricuter: [bid., 
48 (1914), 700. 

PanEeTH: Physik. Z., 15 (1914), 924; Panneru and Horovitz: Z. phystk. 
Chem., 89 (1915), 513; Wiener Akademie Wissensch., 123 (1915), 1819. 

Lacus and Micnwaetis: Kolloid-Z., 9 (1911), 275; 31 (1922), 208. 

Rona and Micuartis: Biochem. Z., 97 (1919), 85. 

GyEMANT: Kolloid-Z., 28 (1921), 103. 

KouuscHtTrer: Z. EHlectrochem., 14 (1908), 49. 

LoTTEeRMOSER and Marria: Ber., 43 (1910), 3613. 

Marria: Kolloidchem. Bethefte, 3 (1911), 85. 

Gouy: J. Phys., (4), 9 (1910), 457; Compt. rend., 149 (1909), 654; 
aun, Phys., (9), 7 (1917), 129. 


CHAPTER X 
ADSORPTION AND CATALYSIS 


By 
Wiper D. BANCROFT 


The increased concentration of reacting substances at the 
surface of an adsorbing catalyst will in itself mean an increased 
reaction velocity; but this factor seems relatively small in most 
cases of contact catalysis, except, perhaps, in some experiments 
with silica where we seem to be dealing primarily with a conden- 
sation in the pores rather than with adsorption. A pressure of 
2,000 atmospheres is not sufficient to make hydrogen and oxygen 
react with measurable velocity at ordinary temperatures.! It is 
not easy to see how differences in concentration can account for 
alcohol decomposing chiefly to ethylene and water in one case, 
and chiefly to acetaldehyde and hydrogen in another. 

What actually happens is that the reacting substances are 
activated as a result of adsorption; but we do not yet know just 
what we mean by activation. When two saturated compounds 
react, the first stage must be a dissociation, which involves the 
breaking of a regular bond, or it must be an addition, which 
involves the opening of a secondary valence? or contravalence. 
One of the problems of contact catalysis is to determine in any 
particular case which bond has been broken, opened, or activated, 
the three terms being synonymous. In aqueous solutions many 
substances are activated by dissociation into ions. 

Langmuir*® considers that adsorption involves the temporary 
union of the adsorbed substance with the adsorbing material; 
but he leaves it undecided for the present just where and how the 

1See also Burpicx: J. Am. Chem. Soc., 44 (1922), 240. 


2For a discussion of primary and secondary adsorption, see. BENTON: 
J. Am. Chem. Soc., 45 (1923), 887. 


3. Am. Chem. Soc., 37 (1915), 1139; 38 (1916), 1145, 2221; 39 (1917), 


1848. 
258 


ee einige tie: 9c, 


ADSORPTION AND CATALYSIS 259 


union takes place, except in a very general way. If a substance 
is adsorbed from aqueous solution by charcoal, for instance, the 
more polar portion of the molecule is assumed to project into the 
water, producing what is known as “oriented” adsorption. 
There are, then, two possible ways in which reaction may take 
place. The adsorbed or captive molecule may be bombarded 
effectively by free molecules, causing areaction. On thisassump- 
tion the reaction will probably take place at the free end of 
the captive molecule. As the bond between the adsorbed sub- 
stance and the adsorbing agent makes or breaks during dynamic 
equilibrium, we have, temporarily, an activated radical and this 
may react with another activated radical or with a neutral 
molecule. On this assumption, the reaction will take place 
chiefly at what is temporarily the captive end of the molecule at 
a moment when the molecule itself is free. Wedo not yet know 
whether it is chiefly the captive molecule or the free radical which 
reacts, or whether both may be considered as activated. Kruyt 
and van Duin? believe that it is the free end of the captive mole- 
cule which reacts. 

If the reaction takes place at a non-polar, or less polar, portion of 
the molecule, this part is turned away from the aqueous phase and 
from the substances dissolved in it. If the two reacting substances 
are more or less polar, it may be that they will be adsorbed in such a 
way that the reacting portions are turned away from each other. From 
this point of view, it is clear why we obtained a negative catalysis at 
first in spite of the adsorption. 

On the other hand, the experiments of D. Berthelot and 
Gaudechon® indicate that ultra-violet light of suitable wave- 
lengths will bring about all the reactions which can be produced 
by catalytic agents; and one has no captive molecules when deal- 
ing with ultra-violet light. It, therefore, seems probable that, in 
most cases, the active masses are the free radicals at the moment 
before or after the molecules are combined with the adsorbing 
material. When substances are activated photochemically, 
there is no question whether a definite or an indefinite interme- 
diate product is formed with the catalytic agent, because hight 
is not a ponderable substance. 


4 Rec. trav. chim. Pays-Bas. (4), 2 (1921), 249. 
5 Compt. rend., 150 (1910), 1169, 1327, 1517, 1690; 151, 395, 478, 1349; 
152 (1911), 262, 376, 522; 153, 383. 


260 COLLOIDAL BEHAVIOR 


While Langmuir considers that an adsorbed substance is 
united chemically with the adsorbing material, he does not mean 
by this, as many people have assumed, that a chemical compound 
of the ordinary type is formed, one described by the law of definite 
and multiple proportions. Langmuir looks upon a coherent 
mass of charcoal as a giant molecule, and when chlorine is 
adsorbed by charcoal, he does not postulate the formation of 
carbon tetrachloride, tetrachloroethylene, hexachloroethane, 
hexachlorobenzene, or anything of that sort. He means that 
the whole of the carbon and the whole of the chlorine are to be 
considered as forming a compound, the composition of this 
so-called compound varying continuously as the chlorine is 
pumped out. If one wants to call such a system a chemical 
compound, it should be called an indefinite compound to differen- 
tiate it from the definite compounds as known to Dalton. 

There is, of course, no theoretical reason why contact catalysis 
should not involve the intermediate formation of definite chem- 
ical compounds, and this is apparently the case when a hydrogen 
peroxide solution reacts with mercury,° when acetic acid is passed 
over heated barium carbonate,’ or when carbon monoxide is 
oxidized in presence of mixed oxides® of cobalt, manganese, etc.; 
but it is important to know whether definite intermediate com- 
pounds are formed or not. If we are dealing with adsorption, 
no definite intermediate compound is formed. It is very prob- 
able that some cases now assumed to involve definite intermediate 
compounds may prove really to be activation by adsorption. 
In the catalytic oxidation of carbon monoxide, it is assumed that 
there is alternate (or simultaneous) reduction and oxidation of 
the catalyst. The oxidation carrier is supposed to oxidize the 
carbon monoxide and to be reoxidized itself by the oxygen of 
the air. Unfortunately, the rate of oxidation of carbon monoxide 
by the higher oxides is relatively slow, so that the alternation 
from one stage of oxidation of the catalyst to another may be a 
negligible factor in the reaction velocity. Bray believes that it 
is useless to try to decide whether, at the dynamic equilibrium, 

6 Brepig and von Anrroporr: Z. Hlektrochem., 12 (1906), 581; von 
AnTROPOFF: J. prakt. Chem. (2), TT (1908), 273. 


7 Squires: J. Am. Chem. Soc., 17 (1895), 187. 
8 Lams, Bray and Frazer: J. Ind. Eng. Chem., 12 (1920), 217. 


ng et 


ADSORPTION AND CATALYSIS 261 


a molecule of oxygen at the surface of the catalyst actually 
changes some of a lower oxide to a higher (and the reverse change 
with carbon monoxide®) or whether the oxygen is merely held 
on the surface in an active condition ready to combine with 
carbon monoxide. In either case, all the processes are taking 
place simultaneously. 

The oxidation of alcohol by air in the presence of osmium tetrox- 
ide can easily be run in two stages,!° because osmium tetroxide 
will oxidize alcohol in the absence of air, and air will oxidize the 
dioxide back to the tetroxide; but we do not know whether the 
single reaction velocities are sufficient to account for the rate of 
catalysis. If not, we shall have to postulate activation of oxygen 
independently of the definite chemical reaction. 

It is a simple matter to account for the decomposition of 
alcohol into acetaldehyde and hydrogen by nickel, and into 
ethylene and water in presence of alumina on the basis of differ- 
ently oriented adsorption. Adkins! has shown, however, that 
differently prepared samples of alumina decompose ethyl acetate 
in different ways. He believes that there is some connection 
between the size of the molecular pores of the alumina and the 
molecular diameters of the decomposition products, while Taylor 
considers that it is a case of selective adsorption on the alumina, 
the ethyl acetate being adsorbed on or by the aluminum atoms 
in one case and on the oxygen atoms in the other case. For 
the moment these are both unproved guesses. 

Pease and Taylor!” have found that the reduction of copper 
oxide by hydrogen takes place practically only at the interface 
between copper and cuprous oxide. Years ago, Campbell! 
recommended the use of palladinized copper oxide in combustions. 
Any advantage of such an arrangement must have been due to 
action at an interface. Lewis!‘ found that finely divided plati- 


® Benton considers that he has proved this. J. Am. Chem. Soc., 45 
(1923), 900. 

10 HormaNnn: Ber. 45 (1912), 3329; 46 (1913), 1657, 2854; 48 (1915), 1588. 

11 J, Am. Chem. Soc., 44 (1922), 385, 2175. 

12 J. Am. Chem. Soc., 48 (1921), 2179; Lanamutr: Trans. Faraday Soc., 
17 (1922), 607. 

13 J. Am. Chem. Soc., 17 (1895), 681. 

147. »physik. Chem., 52 (1905), 310; 55 (1906), 449; J. Am. Chem. Soc., 
28 (1906), 139. 


262 COLLOIDAL BEHAVIOR 


num, silver, and manganese dioxide catalyze the dissociation 
of silver oxide into silver and oxygen. This is probably also a 
reaction at the interface and the same explanation probably 
holds for the action of finely divided platinum and certain metallic 
oxides upon the dissociation of mercuric oxide. 

A striking case of action at an interface, and one which has 
been known for a long time, is the zinc-copper couple of Gladstone 
and Tribe.!® With copper precipitated on, and in intimate con- 
tact with, zinc, it is possible to decompose many of the alkyl 
halides at moderate temperatures. When the couple is made 
from zine dust,'’ the preparation of zinc methyl can easily be 
shown as a lecture experiment. The couple decomposes bromo- 
form, giving methane and acetylene.'* Thorpe’? used the couple 
as a means of reducing nitrates, iodates, and chlorates quantita- 
tively, and, under his direction, Eccles?° used it to determine 
chlorates in the presence of perchlorates, the former being 
reduced to chlorides and the latter not. Devarda’s alloy?! seems 
to be another form of the same thing. It isnot customary to con- 
sider the action of the zinc-copper couple as catalytic, because the 
zinc reacts; but it is catalytic as regards the copper. In aqueous 
solutions the action of the couple is undoubtedly electrolytic and 
it may perhaps always be so. With the organic substances the 
infinitesimal distance between the two metals may counter- 
balance the high resistance of the organic liquid. When copper 
oxide is reduced by hydrogen, we do not know whether this is a 
limiting case of electrolysis or whether there is a special activa- 
tion owing to the hydrogen being adsorbed simultaneously by the 
two substances at the interface. 


* Taytor and Huxerr: J. Phys. Chem., 17 (1913), 565; J. Am. Chem. 
Soc., 44 (1922), 1443; Kmnpatu and Fucus: 43 (1921), 2017; 44 (1922), 
1447. 

© Proc. Roy. Soc., 20 (1872), 218; J. Chem. Soc., 25 (1872), 461; 26 (1873), 
445, 453, 678, 961; 27 (1874), 208, 406, 410, 615; 28 (1875), 208; 30 (1876), 
37; 31 (1877), 561; 33 (1878), 139, 306; 35 (1879), 107, 172, 567. 

LACHMANN: Am. Chem. J., 19 (1897), 410. 

8 Cf. Sargent: J. Phys. Chem., 16 (1912), 407. 

19 J. Chem. Soc., 26 (1873), 541. 

20 [bid., 29 (1876), 856. 

*!Z. anal. Chem., 33 (1894), 113; Atuen: J. Ind. Eng. Chem., 7 (1915), 
522. 


ADSORPTION AND CATALYSIS 263 


Another phenomenon which may be due to an action at an 
interface is that known as promoter action.22. Reduced iron 
is the most effective single catalyst that can be used commercially 
in the ammonia synthesis, but its activity can be increased by 
the addition of small amounts of molybdenum, tungsten, or 
cerium. If these substances form separate phases, they will give 
rise to interfaces; but, if they form solid solutions, the question of 
action at an interface does not arise. While the action of these 
promoters, as they are called, may be at the interface, this is 
probably not the important factor, because the maximum effect 
comes at low concentrations, long before the interface is a maxi- 
mum. While the theory of promoter action has not been worked 
out, a plausible guess is that the catalytic agent activates one 
reacting substance chiefly, and that the promoter activates the 
other. ‘Thus, in the ammonia synthesis, it may be that iron 
activates hydrogen chiefly, so that we have hydrogenation of the 
nitrogen. The molybdenum may tend to activate the nitrogen 
or may increase the activation of the nitrogen, thus causing nitri- 
dation of the hydrogen. Such a state of things is not impossible, 
theoretically. When a dye reacts with the oxygen of the air 
under the influence of light, the light may make the oxygen so 
active that it will oxidize the dye, or the light may make the 
dye active, in which case the activated dye will reduce the oxygen. 
Whether it is primarily the dye or the oxygen that is activated 
depends on whether the effective light corresponds to an absorp- 
tion band for the dye or for the oxygen. Experimentally, it 
appears that, under ordinary conditions, it is apt to be the dye 
which is activated.” 

Promoter action is by no means confined to the ammonia 
synthesis. Pease and Taylor** have collected the data on pro- 
moter action and it appears that the phenomenon is a fairly 
common one. Ipatiew?> found that copper oxide in an iron tube 
is much more effective in causing the hydrogenation of amylene 
than is copper oxide in a copper tube. The Badische Company”® 


22 RipEAL and Tayuor: ‘‘Catalysis in Theory and Practice,” 1919, p. 31. 
23 BrREDIG and PEMSEL: Archiv wiss. Photographie, 1 (1899), 33. 

24 J, Phys. Chem., 24 (1920), 241. 

2% Ber. 43 (1910), 3387. 

*D. R. P. 282, 782 (1913). 


264 COLLOIDAL BEHAVIOR 


states that hydrogenation of fats is accelerated by the presence 
of tellurium. Dewar and Liebmann?’ claim that a mixture of 
nickel and copper oxides can be reduced in cottonseed oil at 
190°C. and will hydrogenate the oil rapidly at that temperature, 
whereas nickel oxide alone requires a temperature of about 250° 
for the reduction. Hochstetter?® found that a mixture of silver 
and copper is more effective for the synthesis of formaldehyde 
from methanol than either metal singly. Maxted?? states that 
bismuth, tungsten, and copper make iron active in the ammonia 
oxidation. Mention has been made of the fact that mixed oxides 
are more efficient in oxidizing carbon monoxide than any of the 
oxides alone. This may be a factor in the behavior of the 
Welsbach mantle, though it has not been proved. 

While there is no activation and, consequently, no contact 
catalysis unless we have adsorption, the converse is not true that 
the catalytic action is greater the greater the adsorption. With 
any given catalyst and any given reaction, the maximum cata- 
lytic activity does not necessarily coincide with the maximum 
adsorption, and usually does not do so, in fact. Adsorption is 
always greatest at low temperatures, whereas there is often very 
little catalytic action below a certain ill-defined temperature. 
There are three possible causes for this. The adsorption of some 
one of the reacting substances or of the reaction products may be 
so great as to interfere with the free flow of matter to and from 
the surface of the catalyst, or it may be that the activation is 
greater at higher temperatures or—which is certainly a factor— 
that the rate of reaction of the activated gases is very much 
greater at higher temperatures. We should, then, have adsorp- 
tion decreasing with rising temperature, activation varying in an 
unknown way with the temperature, and the rate of reaction of 
the activated gases increasing rapidly with rise of temperature. 

Taylor and Burns*® consider that: 


Reaction is the resultant of at least two factors, the adsorption factor 
and the temperature factor. There is evidence as to how these two 
factors operate. For example, it is known that ethylene and hydrogen 


27 U. 8. P.1268692, 1275405: 

US. P2000. P11 0Z89: 

*9 J. Soc. Chem. Ind., 36 (1917), 777. 
80 J. Am. Chem. Soc., 48 (1921), 1283. 


ADSORPTION AND CATALYSIS 265 


can be caused to react by purely thermal means. The temperatures 
required are high, being in the region of 500°. With a nickel catalyst 
present, reaction occurs from room temperature upward, and strong 
adsorption of both gases is shown by nickel. With copper, a tem- 
perature of 150° is required for incipient action and adsorption by this 
metal is much less pronounced than in the case of nickel. Similar 
observations hold in respect to the conversion of carbon monoxide and 
hydrogen by means of nickel and cobalt, the temperature required 
in the use of cobalt, the less efficient adsorption agent, being some 90° 
higher than with nickel for similar rates of reaction. It will, therefore, 
appear that the adsorption capacity is an index of the temperature at 
which reaction can be induced. Where adsorption is strong we have, 
presumably, a more marked or more frequent displacement of the 
stable configuration of the molecule than with a weak adsorption. 
Consequently, a lower temperature will effect interaction. 


On another page Taylor and Burns say that: 


The measurements with active nickel catalysts and with inactive 
nickel obtained by reduction of the oxide at elevated temperatures 
form, we believe, convincing experimental demonstration that the 
destruction of catalytic activity is accompanied by an almost complete 
suppression of adsorption power. 

Benton*! concludes that: 

No connection whatever exists between the extent of secondary 
adsorption and catalytic activity for carbon monoxide oxidation. The 
primary adsorption of carbon monoxide, however, is in exactly the 
same order as the catalytic activity. 


It seems to be certain that adsorption depends essentially on a 
porous structure. H. Briggs*? considers that a smooth or 
vitreous surface has a relatively low catalytic action. Taylor 
and Burns state that 97 per cent of the adsorptive capacity of nickel 
for several gases is destroyed by heating the nickel to 600°. 
Though some sintering does take place, there is no reason to 
suppose that the new surface is only 3 per cent of the old one. 
There has been a change in the nature of the surface. Armstrong 
and Hilditch** have obtained similar, though less striking, results 
with nickel, while Gilfillan*+ has shown that, by calcining 


31 J, Am. Chem. Soc., 45 (1923), 901. 
32 Proc. Roy. Soc., 100 A (1921), 97. 

33 Proc. Roy. Soc., 99 A (1921), 490. 

34 J. Am. Chem. Soc., 44 (1922), 1323. 


266 COLLOIDAL BEHAVIOR 


thoria strongly or by heating it for a long time at a lower tem- 
perature, the oxide can be made practically inert so far as the 
dehydration of alcohol is concerned. Kramer and Reid?* have 
made a very inert thoria in an unexpected way by dropping 
thorium nitrate into ared hot crucible. The rapid decomposition 
gives a very bulky thoria, 35 g. of it occupying a liter. In 
spite of the enormous surface, this thoria was practically inactive 
as a catalyst, presumably because the surface was vitrified and 
not porous. 

Palmer*® could not precipitate copper electrolytically so that 
it would dehydrogenate ethyl or isopropyl alcohol at any tem- 
perature between 200 and 300°. He considers that there are two 
kinds of copper, that obtained by electrolysis being composed 
of cupric copper atoms, while that from copper oxide is a mixture 
of cupric copper atoms and cuprous copper atoms. The two 
hypothetical types of copper are assumed to be heterotopic, 
_ to have different surface lattices, to possess different chemical 
properties,*’ and to adsorb differently. Palmer assumes, further, 
that the ratio of the two types of copper will vary with the reduc- 
ing agent and with the temperature at which reduction takes 
place. Unfortunately for this flexible hypothesis, samples of 
copper prepared by Benton at Princeton by reduction of different 
samples of copper oxide at different temperatures all showed the 
characteristic copper metal lattice when examined by the x-ray 
method at the Research Laboratory of the General Electric 
Company at Schenectady. It is, therefore, not necessary to 
discuss whether electrolytic copper is cupric copper. The whole 
dificulty is undoubtedly that Palmer did not precipitate his 
copper in a sufficiently porous form. 

While the catalytic activity can be reduced practically to zero 
by a change in the structure of the catalytic agent, the same 
result can also be obtained by the action of poisons, as they are 
called. In technical catalytic processes, the great difficulty is to 
keep the catalyst active, as the presence of any one of a number of 
substances even in minute amount will poison the catalytic 
agent and render it inert. Up to a few years ago, this poisoning 

3 J. Am. Chem. Soc., 43 (1921), 882. 


%6 Proc. Roy. Soc., 98 A (1920), 15; 99 A (1921), 412. 
37 Soppy: J. Chem. Soc., 115 (1919,) 23. 


ADSORPTION AND CATALYSIS 267 


of the catalytic agent was considered a most mysterious phenom- 
enon, but the theory of it is now quite satisfactory. Since the 
reaction takes place in or at the surface of the catalytic agent, 
any substance, gas, liquid, or solid which decreases the rate at 
which the reacting substances reach the catalytic surface,*® 
or which prevents them from reaching it,** will decrease the reac- 
tion velocity and may destroy the catalytic action completely. 

Berliner’? has shown that traces of fatty vapors from the air or 
from the grease on the stop-cocks will decrease the adsorption of 
hydrogen by palladium from about 900 volumes to nothing. 
Pollard*! observed a similar decrease with platinized asbestos, 
from about 160 volumes to a negligible amount. Faraday?” 
proved that traces of grease destroy the catalytic action of 
platinum black. Lunge and Harbeck* found that carbon monox- 
ide inhibits practically completely the catalytic action of plati- 
num on a mixture of ethylene and hydrogen. Taylor and Burns‘ 
have shown that this is because carbon monoxide decreases the 
adsorption of hydrogen by platinum black. Working under 
more favorable conditions, Pollard*> has obtained a similar, but 
more striking, result. With platinized asbestos, kept very clean 
from grease, he obtained an adsorption of 160 volumes of hy- 
drogen per volume of platinum. On introducing carbon monox- 
ide the hydrogen adsorption dropped to about 7 volumes, which 
was practically negligible under the conditions of the experiment. 
Taylor and Burns conclude from their experiments that, even 
when the pressure of carbon monoxide does not exceed a few 
centimeters, the platinum surface is probably covered so thor- 
oughly with carbon monoxide that hydrogen and ethylene 
are unable to reach it. They also point out that platinum, which 
holds carbon monoxide so tenaciously, is not a good catalytic 
agent for the reduction of carbon monoxide to methane, whereas 
this reduction takes place readily at a palladium surface, from 

38 Taytor: Trans. Am. Electrochem. Soc., 36 (1919), 149. 

39 BancrorT: J. Phys. Chem., 21 (1917), 734. 

40 Wied. Ann., 35 (1888), 903. 

41 J, Phys. Chem., 27 (1923), 356. 

42 ‘Hixperimental Researches on Electricity,” 1 (1839), 185. 

43 Z. anorg. Chem., 16 (1896), 50. 


44 J. Am. Chem. Soc., 42 (1921), 1285. 
4 J, Phys. Chem., 27 (1923), 356. 


268 COLLOIDAL BEHAVIOR 


which carbon monoxide can be displaced easily by hydrogen at 
ordinary temperatures. 

Schénbein**® pointed out that the hydrides of sulfur, tellurium, 
selenium, phosphorus, arsenic, and antimony act very energeti- 
cally in cutting down the action of platinum on mixtures of air 
with hydrogen or ether. Since he did not realize that an adsorbed 
gas film might keep out other gases, he decided that these hydrides 
must decompose and plate out a solid film on the platinum. This 
hypothesis is not necessary to account for the phenomenon; but 
Schénbein was right in at least one case. The most complete 
experimental study of poisons made so far is by Maxted,*” who 
has shown that hydrogen sulfide is decomposed by platinum 
black with the evolution of hydrogen, and that the “sulfurized”’ 
platinum does not adsorb hydrogen. With varying amounts of 
hydrogen sulfide, both the adsorbing power and the catalytic 
action decrease linearly over a certain range of concentrations. 
When platinum is poisoned by lead, 1 mg. of lead poisons nearly 
9 mg. of platinum, this figure applying only to platinum black 
prepared in a given way and, therefore, having a given ratio 
of active surface to mass. 

Maxted has also made experiments with mercuric chloride, 
mercuric nitrate, and lead acetate on platinum prepared by Loew’s 
method, the catalysis of hydrogen peroxide being taken as the test 
of the poisoning. With mercuric chloride it made little or no 
difference whether the platinum was left in contact with the 
mercuric chloride solution for 30 minutes or for 12 hours. This 
shows that there is no progressive deterioration of the platinum 
and that the change takes place fairly promptly. ‘The curve 
obtained by plotting the catalytic activity of the platinum against 
the poison content is practically linear until at least 70 per cent of 
the original activity has been suppressed. 

Twenty-five years ago, Bredig*® showed that many substances 
poison the action of platinum on hydrogen peroxide solutions. 
The rate of decomposition of hydrogen peroxide by a given 


46 J. prakt. Chem., 29 (1848), 238. 

47 J. Chem. Soc., 115 (1919), 1050; 117 (1920), 501; 119 (1921), 225, 1286; 
121 (1922), 1760. ’ 

48 BREDIG and VON BERNECK: Z. phystk. Chem., 31 (1899), 258; Brepia 
and Ixepa: 37 (1901), 1. 





ADSORPTION AND CATALYSIS 269 


suspension of colloidal platinum is reduced approximately to one- 
half by m/20,000,000 HCN, m/2,000,000 HgClh, and m/300,000 
H.8. Since Sch6nbein*? had shown that these substances 
decrease the catalytic action of the red blood corpuscles on a 
solution of hydrogen peroxide, and since the catalytic agent is the 
organic ferment, hzemase, in the blood corpuscles, Bredig called 
his colloidal metals inorganic ferments. 

It seems probable that the poisons are adsorbed strongly by the 
colloidal platinum and, therefore, prevent the adsorption and 
decomposition of hydrogen peroxide;*® but the only data on the 
subject are those obtained recently by Maxted. 

Pease®! has shown that the vapor from 1 cu. mm. of liquid 
mercury introduced into 100 g. of a reduced copper catalyst 
inhibits completely the reaction between hydrogen and ethylene 
at 0°. Furthermore, the reaction is still extremely slow at 100°. 
The mercury suppresses practically completely the adsorption of 
hydrogen by the copper, but has very little effect on the adsorp- 
tion of ethylene. 

Ueno”? has studied the effect of various additions to the nickel 
catalyst in the hydrogenation of oils. There is nothing in the 
abstract to show whether any theoretical conclusions were 
drawn, so presumably that was not the case. 


All of the six alkali metals act as negative catalyzers; magnesium, 
strontium, calcium, and beryllium retard it; aluminum and cerium are 
negative catalyzers; a large amount of iron also retards it. Manganese 
and cobalt are apparently negative; zinc and cadmium show negative 
reaction. Copper and lead are poisons; mercury retards; silver, 
thallium, bismuth, and antimony are poisons, while vanadium, tin, 
titanium, uranium, and tungsten are not. Gold, platinum, iridium, 
and osmium do not retard the hydrogenation. Sulfur and selenium 
are poisons; but tellurium is not to such an extent. Cyanogen and 
cyanides are strong poisons, asis phosphorus. Boric acid is not negative 
when added to the catalyzer; but is when present in the oil. Presence 
of organic matter has no effect if treated at high temperatures. 


49 J. prakt. Chem., 105 (1868), 202. 

60 Cf. Spnter: Z. physik. Chem., 51 (1905), 702; 72 (1910), 689. 
51 Private communication from Prof. H. 8. Taylor. 

52 Chem. Abstracts, 15 (1921), 1226. 


270 COLLOIDAL BEHAVIOR 


Harned*’ has shown that the rate of adsorption of chloropicrin 
by a charcoal which has been cleaned by washing with chloro- 
picrin is much greater than by a charcoal which has not been so 
cleaned, although the final equilibrium is apparently about the 
same in the two cases. This is analogous to the evaporation 
of water when covered by an oil film. The oil cuts down the rate 
of evaporation very much but has practically no effect on the 
partial pressure of water at equilibrium. 

It is easy to see that a piling up of any of the reaction products 
on the surface of the catalyst will decrease the reaction velocity 
if this hinders or prevents the reacting substances from com- 
ing in contact with the catalytic agent. This has been ob- 
served in the contact sulfuric acid process.*4 The explanation 
that the decrease in the reaction velocity is due to a de- 
creased adsorption of the reacting substances was first given 
by Fink,®> who is the real pioneer in this line. Although the 
reaction between carbon monoxide and oxygen is practically 
irreversible, it occurred to Henry,** nearly 90 years ago, that the 
presence of the reaction product, carbon dioxide, might slow up 
the rate of reaction, and he proved his point by increasing the 
reaction velocity when he removed the carbon dioxide with caus- 
tic potash. Water vapor checks the catalytic dehydration of 
ether®’ and of alcohol? somewhat, and hydrogen cuts down the 
catalytic dehydration of alcohol. In fact, nickel and copper tend 
to dehydrogenate substances in the absence of hydrogen and to 
hydrogenate them in its presence. 

Since the poisoning of a catalytic agent is due to marked adsorp- 
tion, which cuts down the adsorption or the rate of adsorption of 
the reacting substances, and since the presence of sulfur trioxide, 
the reaction product, tends to decrease the rate of reaction of — 
sulfur dioxide and oxygen, it follows that an extremely strongly 
adsorbed reaction product will act as a catalytic poison. In. 

3 J. Am. Chem. Soc., 38 (1916), 1145. 

54 BODLANDER and Koppren: Z. Elektrochem., 9 (1903), 566; BERL: 
Z. anorg. Chem., 44 (1905), 267. 

Re ees and Fink: Z. sl Chem., 60 (1907), 61; Cf. BUNSEN: 
J. Chem. Soc., 25 (1873), 736. 

56 Phil. Mag. (3), 9 (1836), 324. 


57 TpatimFF: Ber. 87 (1904), 2996. 
88 HNGELDER: J. Phys. Chem., 21 (1917), 676. 





ADSORPTION AND CATALYSIS 271 


such a case the extent to which the reaction will run will depend 
on the relative amount of catalytic agent present.*°® If a large 
amount of catalytic agent be added to a mixture which does not 
react perceptibly in finite time in the absence of the catalytic 
agent, the reaction will run to an end or to true equilibrium before 
the catalytic agent is poisoned completely. If there is only a 
small amount of catalytic agent, it will be poisoned very early in 
the course of the reaction and we shall have an apparent equilib- 
rium, reached from only one side, which will vary with the 
amount of catalytic agent. For any given small amount of 
catalytic agent we shall get an apparently definite end point; 
but the value of the end point will vary with the amount of the 
catalytic agent taken. ‘This is called autotoxic catalysis. 

At least one case of this sort has been recognized definitely. 
The amount of splitting of amygdalin by platinum black is small, 
because one of the reaction products is hydrocyanic acid and this 
poisons the platinum black. Instead of working with tightly 
corked flasks, Neilson left the flasks uncorked, and found that 
the evaporation of the hydrocyanic acid allowed the decomposi- 
tion to proceed somewhat farther. 

Since enzymes are poisoned in the same way that colloidal 
platinum is, it seems worth while to consider whether autotoxic 
catalysis will account for some of the peculiarities in enzyme 
action which have puzzled people. If autotoxic catalysis occurs, 
the presence of the reaction products will cause a decrease in the 
reaction velocity even though the reverse reactions are negligible. 
If the poisoning action of the reaction products is sufficient, we 
shall get false equilibria if we start with small amounts of the 
enzymes, while the reaction will run to an end or to true equilib- 
rium if we start with sufficiently large amounts of the enzymes. 
Both of these cases have been observed;*! but no one has yet 
studied the effect of autotoxic catalysis on the reaction velocity. 
This seems a promising field for investigation. 

It has already been mentioned that a sintered catalyst is 
usually practically inert. The question at once arises whether 


59 BancroFT: J. Phys. Chem., 22 (1918), 22. 

60 Nertson: Am. J. Physiol., 15 (1906), 148. 

61 TAMMANN: Z. physik. Chem., 18 (1895), 426; Kast and LoEVENHART: 
Am. Chem, J., 24 (1900), 491; Banorort: J. Phys. Chem., 22 (1918), 39. 


272 COLLOIDAL BEHAVIOR 


it is possible to prevent agglomeration of the catalyst and whether 
there are any accompanying disadvantages. A colloidal solution 
of Bredig’s platinum is fairly unstable, but can, of course, be 
made more stable by the addition of a protecting colloid, such as 
gelatin. Ten years ago, Groh*? showed that the stabilization of 
colloidal platinum by gelatin causes a decrease in the catalytic 
action on hydrogen peroxide, the time for half decomposition 
increasing nearly tenfold as the amount of gelatin increased from 
nothing to 14 per cent. 

All the subsequent work has confirmed the generalization that 
we pay for stabilization by a protecting colloid through decrease in 
the catalytic action. This effect may be masked to some extent 
if the protective colloid increases the dispersion and, therefore, 
the surface of the catalytic agent; or if the protecting colloid is 
itself a catalytic agent.®* 

Iredale®* has studied the effect of protecting colloids on the 
catalytic decomposition of hydrogen peroxide by colloidal plati- 
num and finds that the stronger a substance is as a protecting 
colloid the greater will be its inhibition of catalytic activity. 
The order of inhibiting effect is: gelatin and glue >egg albumin > 
sucrose, the last not appearing to affect the reaction at all. 
One part of gelatin in 20,000,000 of water has a recognizable 
inhibiting action. Iredale has determined inhibition numbers 
for these substances, corresponding to the gold numbers. Taking 
the values for gelatin as 100 in both cases, the gold numbers for 
gelatin, egg albumin, dextrin, and starch are 100, 20, 0.66, and 
0.40, while the inhibition numbers are 100, 20, 1, and 0.33. 

Rocosolano® found that stabilizing Bredig’s platinum with a 
little gelatin decreases the rate of decomposition of hydrogen 
peroxide to about one-third. With sodium lysalbinate the rate 
passes through a minimum with increasing concentration, 
increasing when the effect of the alkalinity begins to count. 
There is no minimum with gum arabic and its effect on the reac- 
tion velocity is much less than that of an equal weight of gelatin, 
which is in harmony with Iredale’s results. 


62 Z. physik. Chem., 88 (1914), 414. , 

63 RipEAL: J. Am. Chem. Soc., 42 (1920), 749. 

64 J, Chem. Soc., 119 (1921), 109; 121 (1922), 1536. 
8 Compt. rend., 173 (1921), 41, 234. 


ADSORPTION AND CATALYSIS 273 


In all these cases the catalytic agent is coated more or less 
completely by the protecting colloid. If, however, we fasten 
the catalytic agent on a rigid support, we shall prevent sintering 
more or less completely and there will be little or no decrease 
in the catalytic action. 

Taylor and Gauger®* have found that the adsorptive power of 
nickel, reduced at 300°, is destroyed if the nickel is heated to 
500°. If the nickel is precipitated on kieselguhr or on diatomite 
brick, it can be heated to, or reduced at, 500° without any change 
in properties. Since there is always agglomeration when nickel 
is reduced at 300°, one might reasonably expect that nickel on 
kieselguhr or diatomite would adsorb more than straight nickel, 
but it is distinctly a surprise to be told that it will adsorb ten 
times asmuch. Pollard*® found that it is impossible to determine 
accurately the amount of hydrogen adsorbed by platinum black 
because the last 30 or more volumes cannot be pumped out 
except by heating to about 300° and at that temperature the 
platinum sinters to such an extent that the adsorption changes 
very much. In other words, adsorption measurements cannot be 
duplicated satisfactorily with straight platinum black. Plati- 
nized asbestos, however, can be heated to 400° without under- 
going any change. Mond, Ramsay, and Shields** obtained 
adsorptions of 110 volumes of hydrogen per volume of platinum 
with their best platinum black; but it is very certain that there 
were at least 30 volumes of hydrogen which they did not get out 
and, consequently, did not measure. ‘This is so near to Pollard’s 
value of about 160 volumes that it would not be safe to claim that 
platinized asbestos adsorbs more or less hydrogen per gram of 
platinum than platinum black in the same state of subdivision. 

Palmer®? has made reproducible copper catalysts by impregnat- 
ing cylindrical rods of china clay with copper formate solution. 
Armstrong and Hilditch’® have investigated the statement by 
Kelber™! that nickel oxide gives a moderately active catalyst when 


6 J. Am. Chem. Soc., 45 (1923), 920. 
67 J. Phys. Chem., 27 (1923), 366. 

6 Phil. Trans., 186 A (1895), 657. 

69 Proc. Roy. Soc., 98 A (1920), 20. 
70 Proc. Roy. Soc., 99 A (1921), 491. 
71 Ber. 49 (1916), 55, 1868. 


274 COLLOIDAL BEHAVIOR 


reduced at about 300° and a very poor catalyst when reduced at 
450°, whereas nickel oxide deposited upon kieselguhr and reduced 
at 450° gives a catalyst which is more active than the straight 
nickel oxide reduced at 300°. This is practically what Taylor 
and Gaugerfound. The unsupported nickel sinters and becomes 
inactive when heated, whereas the kieselguhr does not shrink 
when heated and holds the nickel in place. It is quite possible 
that the nickel film is thinner and has more surface when precipi- 
tated on kieselguhr than when produced from straight nickel 
oxide. 

Armstrong and Hilditch also discuss the statement that 
partially reduced nickel oxide is more active than the same 
oxide when reduced completely. Since a partially reduced nickel 
oxide will consist of a film of metal coating the unreduced core 
of the particle, this will behave like a supported catalyst, 
nickel on nickel oxide, and will, therefore, be of the same type, 
although not of the same degree of activity as nickel upon 
kieselguhr. 

The essential difference between the ‘‘supported”’ catalyst and 
the “protected”’ catalyst is that the platinum is on the outside of 
the asbestos and its rate and power of adsorption are interfered 
with only at the points of contact between platinum and asbestos, 
whereas the gelatin is on the outside of the platinum and interferes 
everywhere with the adsorption of the reacting substances. 
Quite in accord with this view is the observation by Nelson and 
Hitchcock?? that the adsorption of invertase by charcoal or 
alumina does not necessarily affect the rate at which it inverts 
sugar. A difference in rate occurs only when the supported 
catalyst is not distributed uniformly throughout the solution. 

While the high temperature,’* which may occur at the surface 
of the catalyst in exothermal reactions, tends to make the 
catalyst sinter, the actual occurrence of the reaction tends to 
disintegrate the mass and may increase its catalytic action. 
Bone” has shown that a metal surface becomes roughened when 
so-called flameless combustion occurs at it, and it is well known 
that a platinum gauze undergoes similar changes” when used to 


72 J. Am. Chem. Soc., 48 (1921), 1956. 

73 ZeIsBERG: Trans. Am. Electrochem. Soc., 36 (1919), 187. 

74 Phil. Trans., 206 A (1906), 1; J. Franklin Inst., 173 (1912), 101. 
7 Parsons: J. Ind. Eng. Chem., 11 (1919), 541. 


ADSORPTION AND CATALYSIS 275 


oxidize ammonia, thereby becoming a more efficient catalyst. 
Landis’* claims that the style of surface varies with the impurities 
in the ammonia and that a platinum gauze must be activated 
anew if one changes to ammonia from another source, as from 
Haber ammonia to coke oven ammonia or to cyanide ammonia. 
There is no independent confirmation of this last statement. 

For a more detailed discussion of the theory of contact 
catalysis, the reader is referred to the Reports of the Committee 
on Contact Catalysis of the National Research Council, two of 
which have already been published.”7 

Among the important technical processes involving contact 
catalysis are the contact sulfuric acid process, the Deacon chlorine 
process, the Haber ammonia process, the Ostwald nitric acid 
process, the Sabatier hydrogenation process, and the Welsbach 
incandescent process (gas mantle). 


% Trans. Am. Electrochem. Soc., 35 (1919), 300. 
7 Bancrort: J. Ind. Eng. Chem., 14 (1922), 326, 444, 545, 636; J. Phys. 
Chem., 27 (1923), 801. 


CHAPTER XI 
COLLOID CHEMISTRY AND CONTACT CATALYSIS 


By 
Huaeu 8. TAYLor 


Contact catalysis is essentially concerned with a variety 
of phenomena occurring in the chemistry of reactions at surfaces 
or interfaces between two phases. ‘The greater the surface or 
interface the greater is the area in which such chemical reactions 
occur. Hence, as is now well known, since active contact 
catalysts display a high ratio of surface to mass, they are 
employed in the finely divided condition. With the properties 
of matter in a finely divided condition, colloid chemistry 1s 
especially concerned. It, therefore, follows that contact catalysis 
is an important branch of applied colloid chemistry. Upon 
theoretical colloid chemistry it can draw for the principles govern- 
ing the behavior of materials in contact with finely divided 
substances. The connection between the two may, however, 
be more intimate, with mutual advantages accruing to each. 
It is undoubtedly true that on a right understanding of the 
general principles of colloid chemistry the student of contact 
catalysis can base much of his reasoning concerning the phenom- 
ena with which he deals. On the other hand, the conclusions 
of the colloid chemist may be amplified and the bases upon which 
his principles are founded may be broadened by an inclusion of 
the facts which the study of contact catalysis reveals. 

In the majority of contact catalytic actions, the reaction occurs 
at a solid-fluid interface. Of these reactions, by far the larger 
proportion are at a solid-gas interface. We may instance, in 
this regard, many technical catalytic processes, the contact 
sulfuric acid process, the oxidation of ammonia to nitric acid, 
the Deacon chlorine process, the vapor phase oxidation of hydro- 
carbons to yield alcohols, aldehydes, and acids, the hydrogenation 

276 


a 


COLLOID CHEMISTRY AND CONTACT CATALYSIS 277 


and chlorination of organic vapors, the water-gas reaction— 
whereby steam and water-gas react to form carbon dioxide and 
hydrogen—processes of preferential oxidation, and the like. 
Reactions at solid-liquid interfaces are less common but not less 
important, as may be illustrated by the industrial hydrogenation 
of liquid fats at surfaces of nickel or other suitable catalyst. 
A few cases are known where the interface is a liquid-liquid 
interface, the Twitchell reagent for fat splitting undoubtedly 
functioning as an agent for promoting reaction between an aque- 
ous and an oil phase. 

All solids tend to adsorb or condense upon their surface, gases, 
vapors, or liquids with which they are brought in contact. The 
extent of adsorption varies with the nature and physical condi- 
tions of the solid, with the nature of the gas or liquid, their concen- 
trations, and temperature. Classical colloid chemistry has been 
concerned with adsorption by relatively few adsorbents, charcoal, 
silica, alumina, glass, wool, rubber, celluloid, asbestos, meerschaum, 
soils, and afew metals. The study of adsorption from the stand- 
point of contact catalysis extends enormously the range of both 
adsorbents and adsorbates.! Such extension of the field of study 
shows that adsorption isa much more inclusive term than formerly 
believed. It embraces not only physical, capillary condensation 
phenomena, non-specific in character and paralleling the physical 
characteristics of the adsorbate, but also definite associations 
between adsorbent and adsorbate, specific and chemical in 
character, independent of the physical characteristics of the 
adsorbate and similar in every respect to ordinary compound 
formation in stoichiometric proportions. It is these latter 
adsorptions which are more important in contact catalysis; the 
former have been discussed almost exclusively in colloid texts. 

An index of the range of adsorption is obtained by a study of 
poisoning in contact catalysis, for such cases are to be attributed 
frequently to adsorption, on the catalyst surface, of a constituent 
of the surrounding phase which is present only in minute quanti- 
ties and which is held by the adsorbent so tenaciously that it is 
with difficulty removed. Certain cases of poisoning are not due 
to adsorption but to actual chemical reaction with the catalyst, 
resulting in modification of the chemical nature of the catalyst. 

1 Adsorbate is a convenient designation for any adsorbed substance. 


278 COLLOIDAL BEHAVIOR 


The problem of poisoning has been treated very comprehensively 
in recent years and has received quantitative study.? It will 
only be necessary, therefore, to indicate some of the typical cases 
which show the range of adsorption phenomena involved. 
Faraday proved that traces of grease destroy the catalytic 
action of platinum. Mond, Ramsay, and Shields showed that 
mercury behaved similarly. Berliner showed the action of 
grease on the adsorption of hydrogen by palladium. Schénbein 
pointed out that the hydrides of sulfur, selenium, tellurium, 
phosphorus, arsenic, and antimony cut down the activity of 
platinum. Maxted has demonstrated the decrease in the adsorp- 
tion of hydrogen by platinum treated with hydrogen sulfide. 
Maxted also showed that sulfur, arsenic, lead, zinc, and mercury 
behave similarly towards platinum. Henry showed that a reac- 
tion product, carbon dioxide, might retard a reaction by reason 
of its covering up the surface, and Fink showed that this 
was true in the case of sulfur trioxide on platinum in the oxidation 
of sulfur dioxide. Water vapor inhibits the dehydration of 
alcohol; hydrogen inhibits the dehydrogenation of alcohol at 
catalyst surfaces. Water vapor inhibits the catalytic activity at 
iron-molybdenum surfaces in ammonia synthesis, even at 500° C. 
Oxygen inhibits a variety of reactions at tungsten surfaces at 
temperatures as high as 2,000°K., as the researches of Langmuir 
have shown. In solutions, the variety of inhibitors of the decom- 
position of hydrogen peroxide by platinum or enzymic catalysts 
well illustrates the range of adsorption on such surfaces. 

The quantitative study of adsorption by contact catalysts has 
been neglected. Only recently has systematic investigation been 
initiated. The results accumulated, however, warrant energetic 
prosecution of the investigation. Stock, Gomolka, and Heyne- 
mann,* made measurements in the course of their investigations 
on the decomposition of arsine which may indicate some adsorp- 
tion of arsine on the walls of the containing vessel. Measured 
pressures of arsine were admitted to glass vessels. The gas was 


2 Bancrort: J. Phys. Chem., 21 (1917); 734; ‘‘ Applied Colloid Chemistry,” 
McGraw-Hill Book Co., 1921; First Annual Report, Committee on Contact 
Catalysis: J. Ind. Eng. Chem., 14 (1922), 326, 444, 545, 642; Maxrep: J. 
Chem. Soc., 115 (1919), 1050; 117 (1920), 1280, 1501; 119 (1921), 225, 1280. 

3 Ber., 40 (1907), 532. 


COLLOID CHEMISTRY AND CONTACT CATALYSIS 279 


then decomposed by heating. The pressure of residual hydrogen 
was measured. Since 


2AsH; = 3H. + 2As 


the hydrogen pressures thus obtained should be exactly 34 the 
initial arsine pressure. This was not found, as the following 
table of results demonstrates. 


Pressure in millimeters 

















At 15°C: AG 25°C. “7 “At 35°C. 
UNE EA a 714.2 739.2 764.4 
5 iA. ee 1,088 . 5 1,125.9 1,163.8 
a (pHi) — p AsH;....-.. | 11.5 1174 11.5 


In the last line is given the difference between the theoretically 
calculated arsine pressure and that observed initially. Stock 
and his co-workers ascribed this difference between calculated 
and observed values to the deviations of arsine from the gas laws. 
There is a distinct possibility that this deviation is to be ascribed 
partially to adsorption and partially to the operation of molecular 
attraction, since at the temperatures employed arsine is some 
80 to 100° above its boiling point (— 54.8°C.). 

Stock and Bodenstein* demonstrated that the reaction velocity 
measurements on the decomposition of arsine on arsenic surfaces 
were representable by the equation 

of ED ae Ape 
dt 
They explained this as due to a distribution of arsine between gas 
space and surface in accordance with Freundlich’s adsorption 
isotherm. | 


1 
(AsH. 3) adsorbed — k (AsH 3) cae 


They state, however, that the arsine adsorbed is probably too 
small to measure. This certainly needs experimental test, 
_and is probably not entirely correct. 


4 Ber., 40 (1907), 570. 


280 COLLOIDAL BEHAVIOR 


Fink’ measured the adsorption of sulfur trioxide on platinum 
and showed the existence of an approximately unimolecular layer 
of this gas on the metal at reaction temperatures usual in the 
contact process of oleum manufacture. This observation was 
incorporated in the Bodenstein-Fink theory of gas reactions at 
catalytic surfaces. The reaction velocity was assumed to be 
determined by the rate of diffusion of the reactant gases through 
a film of adsorbed resultant, which film was assumed to vary in 
thickness with the partial pressure of such resultant. As Lang- 
muir has pointed out, with only unimolecular layers possible, 
this is not satisfactory. Rather, reaction rate is conditioned by 
the fraction of the surface which is bare of the strongly adsorbed 
gas (SOs3) under the experimental conditions. 

The results of Kuster* and Berl’ are similarly interpretable, the 
catalysts used being vanadium pentoxide and arsenic pentoxide, 
the reaction, however, in each case being much slower than with 
platinum. ‘The researches of L. and P. Wohler and Pliiddemann® 
on the catalytic activity of oxides which might form sulfates as 
intermediate steps in the process, e.g., Fe,O3, Cr2O3, and those in 
which this is not likely, AlzO3, SiOz, TiOe, ete., are worthy of 
study by the student of adsorption in its relation to catalysis. 
These workers established the undoubted influence of the state 
of division of the catalyst on its catalytic activity. They 
showed that gross particle size was not a measure of catalytic 
action. They attempted to measure effective surface by studying 
adsorption of acids, such as acetic and benzoic acids, from solu- 
tions by the various catalysts studied. They reach an adverse 
conclusion in this regard, largely, in the view of the present 
author, by reason of the specificity of adsorption displayed in the 
examples studied, a possibility of which the authors were aware 
but concerning which no large body of available evidence was 
then to be found. 

Langmuir’s researches on the clean-up of gases by filaments 
are too well known to need record here. Those investigations 
and his studies of catalysis at platinum surfaces? have given 


> Z. physik. Chem., 60 (1907), 1. 

6 Z. anorg. Chem., 42 (1904), 453. 

7Z. anorg. Chem., 44 (1905), 267. 

8 Z. physik. Chem., 62 (1908), 641. 

® See especially, Trans. Faraday Soc., 17 (1921), 607, 621. 


COLLOID CHEMISTRY AND CONTACT CATALYSIS 281 


results from which may be deduced a capacity of such surfaces to 
adsorb at a maximum a monomolecular film of various gases. 
Quantitative measurements on platinum, glass, and mica confirm 
this conclusion. 

Systematic study of the quantitative data on adsorption by 
catalysts when subjected to the action of poisons has been carried 
out by Maxted in recent years.!° The decrease in activity caused 
by the poisons, lead, mercury, zinc, sulfur, and arsenic, is directly 
proportional to the concentration of inhibitant from zero con- 
centration up to that producing practically complete inactivity. 
The occlusive power of palladium for hydrogen varies directly 
as the amount of sulfur present as inhibitant. The amount of 
lead, as poison, required to reduce the catalytic activity to one- 
half is very much less than that which reduces the occlusive power 
to one-half its original value. This may be explained by the 
fact that, while occlusion is not confined to the surface, catalysis 
is mainly a surface phenomenon. With metals showing adsorp- 
tion without marked occlusion, there would doubtless be complete 
identity between loss of adsorptive capacity and loss of catalytic 
activity. 

Systematic studies of adsorption of a variety of gases by 
metal catalysts for hydrogenation processes and of metals for 
ammonia decomposition, of oxides for oxidation processes, and 
of a few salts have been made in the Princeton Laboratories.!! 
The measurements have been made over a wide temperature 
range and in a few typical cases over a range of pressures. Ina 
few cases, the intimate parallelism between adsorptive capacity 
and catalytic activity has been traced; Pease’s recent studies of 
the hydrogen-ethylene combination and adsorptions on copper 
and Benton’s recent studies of carbon monoxide and hydrogen 
adsorptions on oxides as accounting for preferential combustion 
of carbon monoxide in hydrogen are cases in point. Gauger’s 

1 Loc. cu., Ref. 2. 

11 Taytor: J. Ind. Eng. Chem., 13 (1921), 75; Tayntor and Burns: 
J. Am. Chem. Soc., 48 (1921), 1273; Taytor: J. Franklin Inst., 197 (1922), 1; 
PrasE and Taytor: J. Am. Chem. Soc., 48 (1921), 2179; 44 (1922), 1637; 
BENnTON: J. Am. Chem. Soc., 45 (1923), 887, 900; Gaucer and TayLor: 
J. Am. Chem. Soc., 45 (1923), 920; Pease: J. Am. Chem. Soc., 45 (1923), 


1196; DouGHERTY one Tayior: J. Phys. Chem., 27 eee 533: JONES and 
Picton: J. Phys. Chem., 27 (1923), 623, 


282 COLLOIDAL BEHAVIOR 


recent studies of nickel-hydrogen isotherms pave the way to 
studies of the thermodynamics of such adsorption processes, 
concerning which practically nothing is now known. Direct 
measurements of heats of adsorption of gases by catalysts are now 
being made. 

The results so accumulated may now be discussed in detail with 
particular reference to the aspects of catalysis and adsorption 
which are involved in such work. 


ADSORPTION ACCOMPANIES CATALYTIC CHANGES 


The general conclusion from the work at Princeton already 
cited is that adsorption is a condition precedent to catalytic 
change. The data obtained by Taylor and Burns on hydro- 
genation catalysts showed marked adsorption of gases which take 
part in hydrogenation processes. Low adsorptive capacities 
were found with relatively inert catalysts. Pease studied this 
relationship in detail with ethylene and hydrogen on copper, 
showing that high catalytic activity was paralleled by high 
adsorptive capacity for both gases. Pease further showed that, 
by suppressing the adsorption of hydrogen by partially poisoning 
the copper catalyst with mercury, the catalytic activity was 
likewise suppressed. Adsorption of both reactants is, therefore, 
a condition precedent to efficient catalysis in this case. Benton 
showed marked adsorption of carbon monoxide and, to a lesser 
degree, oxygen by oxide catalysts capable of effecting the combi- 
nation of these gases. Dougherty and Taylor demonstrated 
the adsorption of benzene vapors by nickel. Taylor, Benton, 
and Dew?? have measured ammonia adsorption on a variety of 
metals which catalyze the decomposition of ammonia. Taylor 
and Beebe!® have shown that hydrogen chloride is adsorbed by 
the copper chloride catalyst of the Deacon chlorine process. 


Toe ForRM OF THE CATALYST AND ADSORPTION 


The extent of adsorption per unit weight of catalyst is deter- 
mined by the method of preparation, distribution on inert sup- 
ports, or by subsequent treatment of the surface by catalyst 
poisons or by heat treatment. 


12 Unpublished work. 
13 Unpublished work, 


COLLOID CHEMISTRY AND CONTACT CATALYSIS 283 


Variation in adsorptive capacity with variation in the methods 
of preparation may be illustrated from the work on copper, on 
nickel, and on an oxide such as cupric oxide. These results are 
strikingly displayed in the following tables. 


ADSORPTIONS ON COPPER 























o | Adsorption per 
ee ; 100 g. Cu at 
Sodunticn Time re- 0°c. and 760 mm., 
of CuO Nature of CuQ quired for cubic centimeters} Observers 
degrees reduction 
Centigrade He C.F 
250 Ignited nitrate...... Few hours 0.2 | 2.85 | Taylor and 
Burns 
200 Kahlbaum’s granules 30-40 hours! 3.0 | 8.00 ! Pease 
150 Kahlbaum’s granules | 4 days 15.5 Taylor and 
Dew 








ADSORPTIONS ON NICKEL 


nd ; Adsorption H»2 
emperature o : . per 100 g. at 
reduction of NiO, | Nature of NiO SEAS oe il 25°C. and 760 Observers 
degrees Centigrade mm., cubic 
centimeters 











300 Ex nitrate 12 hours 47 Taylor and 
Burns 

300 Ex nitrate ? 70 Gauger and 
Taylor 

300 Ex nitrate 2 days 130* Taylor and 
Beebe 


* This catalyst probably more finely divided than the first two. 


ADSORPTIONS ON CuO 


Adsorption per 100 g. CuO at 25°, 760 mm. 





Nature of CuO 





CO2 O2 CO Observer 
Strong ignition of Cu.. 0.015 0.005 0.012 Benton 
Calcination of nitrate. . 0,132 0.000 0.180 Benton 


Precipitation of hydr- 
PORRC etd c sa tycenls's.- eoe2 (0-Orl tL s0O-C,)- + laye (0°C.)4 | Benton 








284 COLLOIDAL BEHAVIOR 


The effect of a catalyst support on the adsorptive capacity 
per unit weight of catalyst is well illustrated by the work of 
Gauger and Taylor with nickel from the calcined nitrate and with 
nickel spread on a diatomite brick. 


He adsorbed per gram of Ni at 750 mm., 
cubic centimeters 


























Catalyst te eee (4 
25°: 80.50. 175°C.1184°C.. |, 200°C. 218°C us2502G: 
Unsiipported (Nigws. oe | 0.69 | 0:635) pee | 0.53 | a 0.84 | 
Ni on diatomite. 4.0.5... a i ee A ee 


The best quantitative data on the effect of poisons on catalyst 
adsorption obtained in the Princeton work are those obtained 
by Pease on copper. Adsorptions of hydrogen and ethylene on 
100 g. of Cu were made before and after the catalyst was poisoned 
with mercury, the quantity of poison being estimated at 200 mg. 


Adsorption at 0°C., and 380 mm., 
cubic centimeters 


He CoH, 
Before Poisoning: a... ae 3.25 8.55 
After poisoning... 2.7. 29.8, aoe 0.15 6.70 


The striking disparity in the influence of the poison on the 
adsorptive capacities of the two gases is worthy of study. The 
hydrogen adsorption is reduced to less than 5 per cent of its 
initial value. The ethylene adsorption, on the other hand, is 
still approximately 80 per cent of its initial value. At the present 
time, we are inclined, taking these data in conjunction with others 
on the effect of heat to be presented below, to attribute this 
phenomenon to differing capacities of surface atoms to adsorb 
hydrogen and ethylene. The mercury vapor, on this hypothesis, 
would be preferentially adsorbed on those portions of the surface 
which have hydrogen-adsorbing capacity. 





COLLOID CHEMISTRY AND CONTACT CATALYSIS 285 


Heat treatment of an active catalyst preparation is now our 
standard method of preparing catalysts with controlled adsorp- 
tive capacity or catalytic activity. Fromavariety of experiments, 
we may choose the following as indicative of the effect produced by 
heat treatment. 

















Adsorption at 
0°C. and 760 mm., 
Catalyst Heat treatment cubic centimeters | Observer 
Ho Gals 
A. Active Cu, | No heat beyond reduction 3.70 8.45 Pease 
100 g. of oxide at 200°C. 
B. A. heated to 450°C. for 1.5 Leis 6.85 Pease 
hours. 
C. Active Ni, | Obtained by reduction of | 35.00 tee Beebe 
24 oxide at 300°C. 
D. D. heated at 400°C. for 4; 16.00 heinchs Beebe 
hours. 











The same abnormal depreciation of the hydrogen adsorption on 
copper is to be noted here as in the poisoning experiments. This 
evidence we would interpret thus: A smaller fraction of the sur- 
face is required for adsorbing hydrogen than for adsorb- 
ing ethylene. The greater adsorptive force required by surface 
atoms in order to hold hydrogen is, in our view, to be regarded as 
possessed by those atoms in the surface which have a greater 
degree of freedom from the normal crystal lattice of the solid 
catalyst. These atoms have a lesser fraction of their electron 
shells surrounded with neighboring copper atoms. They, there- 
fore, possess a greater surface energy. ‘They would also possess 
a higher vapor pressure. With the moderate heat treatment 
accorded to the catalyst in the above-mentioned cases, these 
atoms distill to positions of lesser surface energy more readily 
than do atoms of less freedom in the solid lattice. It is these 
atoms of high surface energy which will be most affected by heat 
treatment; they should be the preferred positions of attachment 
of catalyst poisons. 


286 COLLOIDAL BEHAVIOR 
Tue SpEcIFICITY OF CATALYTIC ADSORPTION 


Freundlich points out! that: 


Since, in adsorption by charcoal, the physical characteristics of the 
adsorbed gas are of far more importance than the specific effect between 
gas and adsorbent, it is not remarkable that also with adsorption by 
different adsorbents the influence of the special properties of the adsorb- 
ent is strongly suppressed (stark zurucktritt). It can be said with a 
certain approximation that oftentimes gases are adsorbed, independ- 
ently of the nature of the adsorbent, in the order of their compressibilities. 


This thesis is entirely inapplicable to catalytic adsorption. 
The ratio for the adsorption of two gases by adsorbents A, B, C, 
2 


etc., which, on the basis of Freundlich’s statement, would be 
approximately constant for each adsorbent, A, B, C, etc., may 
vary quite widely for catalytic adsorbents. The large differences 
in the ratio of adsorption of carbon monoxide at 0°C. to carbon 
dioxide at 0.°C., obtained by Benton, show the specific nature of 
carbon monoxide adsorption at this temperature for a variety of 
oxide catalysts. 


| : 
_ aCO yer | Hopcalite | CuO | MnO, | Fe,0; V.0; SiO. 
| 


DEC) Vis Orage 0.37 0.22 0.09 0.14 0.08 














The same ratio at 25°C. for a few metallic catalysts is pia 
able from Burns’ measurements. 


COM mee Co Fe Pd | Pt Black 


| 
COsmec, loin | a nk er 
Ble ac a Weas tei 3.6 2.8 288 10.6 


| 


It is very evident, since this ratio varies from 0.1 to 300, that 
the Freundlich relation is entirely untenable for such cases as we 
are dealing with here. It has only a very circumscribed appli- 
cability, namely, to chemically inert adsorbents and to easily 
liquefiable gases. A most striking case of the specific behavior of 
catalytic nickel is to be found in Freundlich’s book (page 203) in 


14“ Kapillarchemie,’”’ 2nd ed., 1922, p. 178. 

















COLLOID CHEMISTRY AND CONTACT CATALYSIS 287 


his discussion of some unpublished work by Zisch on the decom- 
position of nickel carbonyl at nickel surfaces. As Freundlich 
points out, one might expect, on the basis of the higher critical 
temperature of nickel carbonyl as compared with carbon 
monoxide, amuch higher adsorption. Actually, carbon monoxide, 
even in minute quantities, exerts a powerful retarding action on 
the decomposition, indicating marked preferential adsorption. 
Our present knowledge with respect to the structure of nickel 
carbonyl and its stable configuration on the basis of the Lewis- 
Langmuir-Kossel theory of structure immediately suggests 
the chemical reasons for this specificity of adsorption, unex- 
plainable on the basis of physical characteristics. Other striking 
variations in ratio of adsorbed gases are to be found in the records 
of the Princeton work. Consideration of the preceding section 
on the influence of catalyst poisons and of heat treatment on 
adsorptive capacity will show, furthermore, that the ratio of 
adsorption of gases by a single catalyst is also variable with 
variation in the method of preparation and of treatment of the 
catalyst. The rule as to non-specificity of adsorbents must be 
discarded when cognisance is taken of the data on catalytic 
adsorbents. 


SPECIFICITY OF ‘ADSORPTION AND SPECIFICITY OF CATALYTIC 
ACTIVITY 


The influence of specific adsorption in determining specific 
catalytic activity is best demonstrated by work dealing with the 
preferential catalytic combustion of carbon monoxide admixed 
with hydrogen. As is well known, metallic oxides may be used 
to catalyze the combination of carbon monoxide and oxygen 
present in equivalent concentrations ina large excess of hydrogen. 
The mechanism of this preferential oxidation is at once apparent 
from the adsorption ratio of the two gases at atmospheric pressure 
on various oxides at —79°C., as determined by Benton. 


aCO Hopcalite | MnO.| CuO | Co.05] Fe2O3) V205 | SiOz 

















on? —79°) a4 100 | 34 19 35 Lag) 28 




















288 COLLOIDAL BEHAVIOR 


For exact comparison with preferential combustion data, adsorp- 
tions at low partial pressures of carbon monoxide should be 
compared with those of hydrogen at approximately atmospheric 
pressure. The results cited, however, show marked preferential 
adsorption of carbon monoxide. With metals, the preferential 
nature of the combustion process is less pronounced. With 
nickel and platinum, the hydrogen is freely consumed; with 
copper, a fair preferential combustion may be attained. Note 
the following data on adsorption ratios of the two gases at various 
temperatures and atmospheric pressure, and contrast them with 
the oxide data. 


a0 Ni Pt Black Cu 


0.87 (184°) 3.3 (100°) . 12. 








The data cited are also of interest in connection with the problem 
of specificity of adsorbent discussed in the preceding section. 


VARIATION OF ADSORPTION WITH PRESSURE AND THE HBAT OF 
ADSORPTION 


As is well known, the variation of adsorption with pressure 
on adsorbents, such as charcoal, is approximately given by the 
Freundlich equation 


where a is the amount adsorbed, and k& and n are constants, the 
latter being always equal to, or greater than, unity. 

The data on the variation of adsorption with gas pressure 
with metallic catalysts as adsorbents are few; some of these, 
however, show striking characteristics. Gauger and Taylor’s 
data on the adsorption isotherms of hydrogen on nickel are the 
most completely studied thus far. The curves obtained at a 
variety of temperatures from 25° to 305°C. show the characteristic 
shape of normal adsorption isotherms, so far as absence of 
discontinuities indicative of compound formation are concerned; 
they show, however, this distinction, that at a certain pressure at 


COLLOID CHEMISTRY AND CONTACT CATALYSIS 289 


each temperature a definite saturation capacity of the surface 
is apparently reached. This saturation capacity is reached at 
very low partial pressures, 40 mm. at 25°C., and approximately 
250 mm. at 305°C. Beyond these pressures, further increase 
in gas pressure up to atmospheric pressure (7.e., 76% = 19 fold 
increase in pressure at 25°C.) adds to the amount of gas adsorbed 
so little as to be within the error of measurement. The same 
observation is true in the recent results of Pollard,’ employing 
hydrogen, and, to a less extent, carbon monoxide on platinum. 
The amount of adsorbed hydrogen in this case does not sensibly 
increase beyond a gas pressure of 100 mm. Pease’s data on the 
adsorption of hydrogen by copper show asimilar if less pronounced 
attainment of saturation capacity. The adsorption of hydrogen 
at 380 mm. pressure was 90 per cent of that at atmospheric pres- 
sure. Similar behavior with respect to carbon monoxide on copper 
is shown in some unpublished measurements of Jones and Taylor 
on the adsorption isotherms of carbon monoxide and carbon 
dioxide on copper at 0° and 80°C. Published work on adsorbents 
of the charcoal type has not indicated the attainment of 
saturation capacity of the surface, even at pressures well beyond 
atmospheric pressure. A further distinction is also noticeable. 
Gauger and Taylor’s results show that the adsorptive capacity 
of hydrogen on nickel at saturation is, at 305°C., as much as 60 
per cent of the saturation capacity at 25°C. Some recent 
data obtained by Dew on copper show adsorptions of hydrogen 
in the ratio of 10 to 8.7 at 0° and 110°C. and atmospheric pressure. 
Contrast this with the data concerning adsorption on charcoal. 
The adsorption of carbon monoxide on charcoal at 400 mm. and 
46°C. is only 8 per cent of that at —78°C., this temperature 
interval being about the same as that obtaining in Dew’s case 
and less than one-half of that recorded above with nickel and 
hydrogen. ‘The adsorption of carbon dioxide on charcoal at 
150°C. and atmospheric pressure is less than 7 per cent of that 
at —78°C. ‘These striking differences, both in the pressures at 
which saturation is attained and in the variation of adsorption 
with temperature, are undoubtedly of fundamental importance 
in the study of catalytic adsorbents. 


1 J. Phys. Chem., 27 (1923), 365. 


290 COLLOIDAL BEHAVIOR 


Data on adsorption isotherms may be utilized to evaluate the 
heat of adsorption of gases on the adsorbent surface. Gauger 
and Taylor, using the minimum pressures at which saturation is 
reached at the several temperatures, and substituting these in the 
equation 
windlass lo Ps 
Pa Tg pe 
obtained a value for \ the heat of adsorption of 2,500 cal. This 
calculation is in error, since the equation should be applied (see 
Freundlich, second edition, page 182) to the pressures P; and Peat 
which equal amounts of gas are adsorbed. The data of Gauger 
and Taylor do not lend themselves readily to such computations 
if accuracy is desired, as the pressures at which equal amounts of 
gas are adsorbed at different temperatures are small and conse- 
quently most liable to error. From the best available data, 
however, calculated in the correct manner, a value for the isosteric 
heat of adsorption of 15,000 + 3,000 cal. was obtained. 

In the meantime, a direct determination of the heat of adsorp- 
tion of hydrogen on nickel has been carried out by Beebe and 
Taylor. We shall report elsewhere’* the details of these measure- 
ments, It will suffice here to say that, on a freshly prepared, 
highly active sample of nickel, a heat of adsorption equal to 
13,500 cal. per mol was obtained. It is to the magnitude of this 
value that we wish to draw special attention. 

It is at once evident that there is a wide deviation in this case 
between the heat of adsorption and the heat of liquefaction of 
hydrogen, which latter cannot be much greater than 450 cal. per 
mol. Further, a little consideration will show that it is to this 
abnormally high heat of reaction that the characteristic curves of 
the nickel-hydrogen isotherms are to be attributed. From 
the isotherms it may be shown that, with a given sample of 
nickel, 8.7 ec. of hydrogen were adsorbed at 25°C. and 40 mm. 
pressure. Utilizing the directly observed value for \ = 13,500 
cal., we may now calculate at what pressure 8.7 cc. of hydrogen 
will be adsorbed at any other temperature. Thus, for 7 = 
184°C., we have 


13,500 = 


h = 4.57 


4.57 X 208 X 457 |, Ps 
(457 — 298) © 40 


16 J, Am. Chem. Soc., 46 (1924), 43. 





COLLOID CHEMISTRY AND CONTACT CATALYSIS 291 


whence P, = 71,300 mm., approximately; or, somewhat less than 
100 atmospheres is the pressure at which the nickel surface will 
be covered with 8.7 cc. of hydrogen gas at 184°C. But Gauger’s 
measurements show that already at 150 mm. pressure 8 cc. of gas 
are adsorbed. A further 100 atmospheres, therefore, only 
produce an additional adsorption of 0.7 cc., a result in entire 
agreement with that found experimentally, namely, that within 
the range 150 to 760 mm. at 184°C., and in the range 40 to 760 
mm. at 25°C., the adsorption was, within the error of measure- 
ment, constant. In a similar manner, it may be calculated 
that at 305°C. it would require some 2,540,000 mm. or some 3,000 
atmospheres gas pressure to cover the surface with 8.7 cc. 
adsorbed gas. In other words, it is tothe abnormally high heat of 
adsorption that the observed independence of adsorption with 
pressure is to be attributed. In confirmation of this observation, 
it is interesting to note that Mond, Ramsay and Shields’ measure- 
ments of the heat of adsorption on platinum of hydrogen gave a 
value of 13,760 cal. per mol of adsorbed gas. Langmuir estimates 
the heat of adsorption of carbon monoxide on platinum at 30,000 
cal. ‘These high values are consonant with the shape of Pollard’s 
curves for the adsorption of these gases on platinum. The data 
of Jones and Taylor on carbon monoxide are suggestive of the 
same high value for heat of adsorption. A calculation from two 
pairs of isosteres gives a value of 6,500 + 300 cal. This is 
markedly higher than the value for the heat of liquefaction, which, 
on the basis of Trouton’s rule, \ = 227';, should be 22 X 81 = 
1,780 cal. It is interesting to note that from Pease’s data on 
ethylene, a gas whose isotherm at 0°C. is much more reminiscent 
of isotherms on charcoal, the value deduced from the isosteres 
at 0 and 20°C. (480 and 760 mm., 5.5 cc. adsorbed) for the heat of 
adsorption may be calculated to be 3,750 cal., which is exactly 
what would be deduced from Trouton’s rule for heat of vaporiza- 
tion. From the isosteres at lower pressures, higher heats of 
adsorption are calculable. From the isostere for 3.85 cc. (200 
and 380 mm. approximately) the calculated value is about 5,100 
cal. 

It is evident, therefore, that in the investigation, on the one 
hand, of adsorption isotherms of various catalyst-gas systems 
and, on the other hand, in the direct determination of heats of 


292 COLLOIDAL BEHAVIOR 


adsorption, we have two powerful instruments with which to 
examine further into the mechanism of catalytic action. Both of 
these instruments are being intensively employed. 

We regard the slight variation of adsorption with pressure 
after the initial strong adsorption at the lower partial pressures 
in several of the cases studied as the strongest evidence in 
favor of Langmuir’s theory of a unimolecular layer. ‘There is 
evident little or no tendency to build up layers of adsorbed 
molecules on such surfaces. Indeed, the results at the higher 
temperatures suggest that considerable pressures may be neces- 
sary before even the surface is covered with a layer one molecule 
deep. With ethylene and ethane at 0°C., in Pease’s experiments, 
and with carbon dioxide at 0°C. in Jones and Taylor’s experi- 
ments with copper, there is possible evidence of liquefaction 
phenomena in addition to the specific adsorption of gas-solid. 

It is interesting to obtain evidence of this from another direc- 
tion in which the difficulties associated with the definition 
of the surface are absent. ‘This is so in a recent publication by 
Iredale,!7 who has investigated the adsorption of methyl acetate 
vapor on liquid mercury by determining the change of surface 
tension of mercury with varying partial pressures of the vapor. 
The following table shows this variation from zero pressure to 
227 mm., the saturation pressure at the temperature employed 
(2650.). 


nn nn rttEtUIEEEEE ya SESE 








V.P. | 0 19 62 | 109 | 137 | 157 [227 min 
eee ee 
¥ 472 | 444 | 423 | 419 | 418 | 417 | 412 dynes/ 
| 370 sq. cm. 


It will be noted that from 62 mm. up to the saturation pressure, 
there is only a slight variation in surface tension with change 
in vapor pressure. This points to the attainment of an approxi- 
mately monomolecular adsorbed layer at less than one-third of the 
saturated vapor pressure. Iredale has calculated the amount 
adsorbed at 62 mm. as of the order of 4.5 X 10-8 g. of methyl 
acetate per square centimeter of mercury. This is equivalent to 


17 Phil. Mag. 45 (1923), 1088. 


COLLOID CHEMISTRY AND CONTACT CATALYSIS 293 


0.37 X 10'° molecules per square centimeter, or to an area of 
approximately 27 X 10—!® sq. em. per molecule, which is com- 
parable with that deduced by Langmuir for the area occupied 
by such esters when oriented at a water surface. 


ADSORPTION AND CatTALytTic ACTIVITY 


This abundance of evidence as to the existence of adsorption 
with catalytic materials must not, however, be taken to imply 
that the existence of adsorption is the sole criterion of catalytic 
change. On the contrary, abundant evidence is forthcoming 
that adsorptive capacity is no sufficient criterion of catalytic 
efficiency. ‘The poor catalytic properties of charcoal and silica 
gel constitute one such piece of evidence. Furthermore, the 
specificity of catalysts, even when adsorptive capacity for 
reactants and resultants is demonstrated, is yet another line of 
evidence. All of the metals listed in the subjoined table show 
measurable adsorptions of the reacting gases in the reactions 
indicated. They display, however, the marked divergencies 
which are noted in the table. 








Reaction | Catalysts | Non-catalysts 
(SRS S G1 Ss i oo) 0 0 ear | Ni, Co, Fe, Pd Cu, Pt 
O10 SS Se Se ee nr Ni, Co, Fe Cu, Pt 
CoHe + 3H, = CeHie PME Pedra atin nets ee aici a) vey eve: cs 5 1 





Ni | Cu 


It is elsewhere suggested that orientation of the adsorbed 
molecules may account in part for such specificity. It seems 
possible to picture the second reaction occurring if the metal-gas 
linkage be Me—CO in the catalytic reactions and Me — OC: 
in the non-catalyzed reactions. As yet, there seems to be no 
evidence either way in this regard. 

While, however, it is certain that orientation may in some 
cases be called in to assist in the explanation of mechanism, it is 
equally certain that orientation in adsorption is not sufficient to 
account for all such specificity of catalytic action. It will be 
generally agreed that such organic molecules as formic acid or the 


294 COLLOIDAL BEHAVIOR 


esters will be adsorbed to the catalyst at the —C = O grouping. 
And yet, as the researches of Adkins have effectively demonstrated, 
it is possible to alter, almost at will, the nature of the decomposi- 
tion produced in a body so adsorbed by suitable alteration of the 
catalyst employed. 

The conclusion seems inevitable that the cause of specificity 
lies in the disturbance exercised by the process of adsorption on 
the configuration of the adsorbed molecule. We have seen already 
that there are evidences of profound change in the abnormally 
high thermal magnitudes associated with certain typical activat- 
ing adsorptions. The adsorption of hydrogen by nickel, as the 
heat of adsorption reveals, is no such small disturbance of the 
electronic forces of the molecule as is involved in a simple con- 
densation process. The disturbance caused is more deep-seated. 
When no such deep-seated disturbance of the molecule occurs, 
as in the case of adsorption by charcoal and silica gel, values 
more closely approximating heat of condensation obtaining, 
little catalytic activity is manifest. 

We look, therefore, towards an explanation of catalysis in the 
influence exercised by the adsorbent on the configuration of the 
adsorbate. By the adsorption, the whole electron field of the 
adsorbed molecule must be altered. The extent of this alteration 
must be revealed in part in the measurement of the energy changes 
involved. The actual alteration achieved must be determined by 
the nature of the adsorbent. We can exemplify this in the com- 
paratively simple case of the decomposition of ammonia at metal 
surfaces where variation in the nature of the decomposition and 
in the ease of decomposition is determined by the nature of the 
metal constituting the surface. At a sodium surface sodamide 
is formed and hydrogen set free. At copper surfaces ammonia 
is freely adsorbed but with difficulty dissociated. With nickel, 
and still more so with iron and with iron-molybdenum, dissocia- 
tion into elementary constituents occurs. Undoubtedly, the 
interplay of electrons as a result of the adsorption association 
is responsible for the divergencies. 

In the more complex cases of the organic molecules, the nature 
of the decomposition products must be determined by the nature 
of the changes in configuration caused by the attachment, That 
such attachments are capable of effecting a pronounced change 


COLLOID CHEMISTRY AND CONTACT CATALYSIS 295 


throughout the molecule is well known to all chemists in a some- 
what different guise. The introduction of a chlorine atom into the 
final methyl group of all the fatty acids exercises an influence on 
the molecule which is transmitted throughout the length of the 
hydrocarbon chain and is revealed in the extent of dissociation 
of the acid. Thus, the acidic hydrogen of chloracetic acid is 
much less firmly attached to the anion than is the hydrogen ion 
in acetic acid, as 1s revealed by its pronouncedly greater degree of 
dissociation. ‘The same is true with propionic, butyric, and the 
higher acids. On the other hand, the substitution of an —NHz2 
grouping for —Cl has the opposite effect. It does not seem 
unreasonable, therefore, to postulate that the nature of the 
catalyst should exercise an influence on the adsorbate which 
is transmitted throughout the length of the adsorbed molecule; 
so that, in the one case, for example, with formic acid, the nature 


of the linkage should promote a O = Cu, _ split and in the 
OH 
_/OH 
other case a O = C split. 
Pa Sa 


ADSORPTION BY CATALYSTS IN LIQUID SYSTEMS 


This field is just beginning to be explored. Rideal!* in a study 
of the hydrogenation of the sodium salts of cinnamic and phenyl 
propiolic acids in aqueous solution at a palladium sol surface has 
attempted to demonstrate the extent of such adsorption. He 
showed that a sol protected by 0.2 per cent gum arabic undergoes 
aggregation when treated with the sodium salts. Of a sol so 
aggregated, 10 mg. were filtered through a small filter and washed 
into a tube connected to a hydrogen burette. The aggregated 
sol and filter paper adsorbed 4.35 cc. of hydrogen at 25°C. A 
duplicate filter paper through which 0.1 N sodium phenyl pro- 
piolate has been filtered required a further 1 cc. of hydrogen. 
Of the sol untreated with salt, 10 mg. adsorbed 1.53 cc. hydrogen. 
Hence, Rideal concludes, the sol had adsorbed salt equivalent 
to 4.35 — (1 + 1.53) = 1.82 cc. hydrogen. This corresponds 


18 Trans. Faraday Soc., 19 (1923), 90. 


296 COLLOIDAL BEHAVIOR 


approximately to 1 molecule of salt to 2 atoms of palladium, 
which may or may not be significant. 

A number of investigations on preferential hydrogenation in 
liquid systems point definitely to the existence of preferential 
adsorption of one constituent of a mixture by the catalysts of 
hydrogenation. Moore, Richter, and van Arsdale!® showed that 
the more unsaturated glycerides were hydrogenated preferentially 
to the glycerides containing only one double bond. Quite 
recently, Richardson, Knuth, and Milligan?° have confirmed this 
conclusion, showing that the preferential nature of the process 1s 
even more pronounced than had been previously believed. A 
newer method of analysis of the hydrogenated product revealed, 
in a typical case, the following percentages of saturated, oleic 
and linolic acid glycerides in the oil before and after hydrogenation. 














Percentages 
Cottonseed oil 
Saturated | Oleic | Linolic acids 
Before hydrogenation............. | 22°77 27.5 49.8 
After*hydrogenation:...-16:. sea | 24.0 67.1 | 8.9 





It is evident that, in this experiment, the hydrogenation was, 
practically exclusively, hydrogenation of linolic acid glycerides 
and negligible hydrogenation of oleic acid compounds. ‘This 
would indicate almost exclusive adsorption of the more highly 
unsaturated glycerides at the nickel surface. The authors found 
that the selectivity of the hydrogenation appears to be more 
marked with increasing amounts of catalyst and with increasing 
temperature up to an optimum in the neighborhood of 200°C. 
Quantitative measurements on preferential adsorption should 
prove very interesting in this case. As Bancroft has already 
pointed out,?! there are almost no quantitative data on selective 
adsorption. An intensive study of the field will be fruitful 
alike to colloid chemistry and contact catalysis. 

19 J, Ind. Eng. Chem., 9 (1917), 541. 


20 A.C. S., September meeting, 1923, Milwaukee, Wis. 
21“ Anplied Colloid Chemistry,’”’ McGraw-Hill Book Co., 1921, p. 73. 





CHAPTER XII 
SENSITIZATION BY MEANS OF HYDROPHILE SOLS 


By 
HERBERT FREUNDLICH 


One occasionally meets with the complaint that, although 
colloid chemistry is undoubtedly of the greatest importance 
in many biological and technical problems, yet its employment 
does not result in as rapid a solution of these problems as might 
be expected. We forget, however, that most of the applications 
of colloid chemistry deal with questions upon which little experi- 
mental work has been done. In the laboratory we generally 
use sols and gels which contain essentially one type of micelle only. 
This produces a decided simplification of the exceedingly com- 
plicated phenomena of colloid chemistry. But in natural and 
industrial processes we deal, in nearly every case, with systems in 
which two or more types of colloidal particles are present, such 
as blood, lymph, bread, cheese, wood, or soil. This renders our 
problems much more difficult for the technic necessary in separat- 
ing particles of different nature from one another has scarcely 
been developed. Furthermore, we have only recently begun to 
understand the reciprocal influences exerted by the particles of 
such systems. 

As one example of such reciprocal influence, the protective 
action exerted by many hydrophile sols on a hydrophobe sol is 
well known and has found many applications. When gelatin is 
added to a gold sol (prepared by the addition of formaldehyde 
to a solution of gold chloride), it becomes much more difficult 
to precipitate by the addition of electrolytes.1 For example, 
in a gelatin-free sol a concentration of 20 millimols of sodium 
chloride is sufficient to cause a change from red to blue within 
5 minutes. But when the gold solution contains about 0.01 mg. 
of gelatin to 0.6 mg. of gold, no coagulation is observed, even 
with a concentration of 200 millimols of sodium chloride. 

297 


298 COLLOIDAL BEHAVIOR 


Many other hydrophile sols, as albumin, casein, gum arabic, and 
dextrin, act in the same manner. Not only metal and sulfide 
sols, but also colloidal dyestuffs, as Congo red, can be protected. 
The particles of the hydrophile sol must always be present in a 
certain excess over that of the hydrophobe in order that a pro- 
tective action may take place. For smaller concentrations of 
the hydrophile sol, a surprising phenomenon often occurs, in 
that the particles of the hydrophobe colloid are either coagulated 
or made more sensitive to the effect of electrolytes. A smaller 
concentration of a given electrolyte is then required for coagula- 
tion than is necessary in the absence of the hydrophile sol. 

This increased sensitivity produced by hydrophile sols has 
been known for a long time,” *» 4 but only recently has it been 
investigated closely and employed for various purposes. In 
the following pages an attempt is made to assemble our present 
knowledge of the nature and use of this phenomenon. 


Tur NATURE OF THE PROCESS 


The increased sensitivity of ferric oxide sols after addition of 
albumin, which was discovered by Pauli and Flecker,* has been 
studied more closely by Brossa and Freundlich. This pair of 
colloids affords a good example of the process in question. 

The customary ferric oxide sol is used, obtained by the dialysis 
of a ferric chloride solution containing ammonium carbonate. 
After mixing this with a carefully prepared solution of electrolyte- 
free albumin, a solution is obtained which is apparently the 
same as that of an ordinary ferric oxide sol of the same concen- 
tration. One difference is that the albumin ferric oxide sol may 
appear a trifle more turbid, in which case it will foam, a behavior 
not exhibited by the pure ferric oxide sol. In contrast to the 
latter, it is much more sensitive towards electrolytes, being 
coagulated by far smaller concentrations than the pure water sol. 

The experiments in Table I were carried out in the customary 
manner. To 5 cc. of the albumin ferric oxide sol, 1 cc. of elec- 
trolyte solution was added. After vigorous shaking, the mixture 
was allowed to stand for 2 hours and the extent of coagulation 
noted. The data refer to the electrolyte concentration after 


SENSITIZATION BY MEANS OF HYDROPHILE SOLS 299 


dilution of the sol. Experiments were conducted at room 
temperature. 


TABLE I 


Albumin sol dialyzed for 5 days. Concentration 6.72 g. per liter; 50 cc. 
mixed with 4.5 cc. ferric oxide sol containing 17.0 g. ferric oxide per liter. 
Pure ferric oxide sol used for comparison: 50 cc. water and 4.5 cc. ferric 
oxide sol. 











Concentration of NaCl,} Albumin ferric oxide | Pure ferric oxide 
millimols per liter sol sol 
0.78 Clear, not flocculated Clear, not flocculated 
1.56 Very turbid Clear, not flocculated 
3.13 Completely flocculated | Clear, not flocculated 
6.25 Completely flocculated | Clear, not flocculated 
12.50 Completely flocculated | Clear, not flocculated 
25.00 Turbid flocculated Clear, not flocculated 
50.00 Somewhat turbia Completely flocculated 
100.00 Somewhat turbid Completely flocculated 


The coagulation values for the mixed sols lie between 0.78 
and 1.56 millimols, or about 1.2 millimoles, while that of the 
pure sol is about 37 millimols. Such experiments can be readily 
duplicated. The behavior of the albumin ferric oxide at higher 
concentrations of sodium chloride will be discussed briefly 
further on. At first it plays no part in the increased sensitivity 
of the sol. 

The albumin ferric oxide sol as well as the untreated ferric 
oxide sol is charged positively and its micelle travel to the nega- 
tive electrode. The ease of coagulation depends on the nature 
of the anion, its valence,® and its capacity for being adsorbed. 
As a rule, the increased sensitivity manifests itself more strongly 
with the weaker coagulating monovalent anions, such as Cl, NOs, 
etc. With the more strongly coagulating polyvalent anions, 
the effect is less pronounced and is sometimes entirely absent. 
Table II shows the degree of increased sensitivity produced by 
some heavy anions. 


300 COLLOIDAL BEHAVIOR 


TasiE II 


For sodium chloride and potassium sulfate the same sol was used as in 
Table I. For the other two a sol prepared as follows was employed: albumin 
sol dialyzed 7 days; concentration 7.8 g. per liter; 50 cc. mixed with 4 cc. 
of the same ferric oxide sol used in Table I. 











Coagulation value of Coagulation value of 
Electrolyte : ; : : ; 
albumin ferric oxide sol pure ferric oxide sol 
Sodium chloride....... 14) 37.00 
Sodium salicylate...... 0.29 2.30 
Soul Ura ve. . ae ea ee 0.18 Oss 
Potassium sulfate...... 0.29 0.59 





For the production of this phenomenon it is necessary in most 
cases that the sol be thoroughly free from electrolytes. In the 
research described above this was accomplished by a long period 
of dialysis, the albumin solution being protected from bacterial 
action by a layer of toluene. A procedure that can be recom- 
mended as much more convenient and rapid is that worked out 
by the Elecktro-Osmose Gesellschaft.’ An electrolysis cell of 
stoneware with a rectangular cross-section made in three separate 
parts A, M, K, Fig. 1, is used. The apparatus is held together 
by a screw. The middle part M is covered on both sides by 
membranes so that the apparatus is divided into three cells. 
The sol is placed in the middle cell between the membranes, and 
the electrodes are placed in the two outer cells. The cells are 
filled with pure water constantly renewed by a suitable device. 
It is important that different membranes be used on the two sides, 
a positive at the anode and a negative at the cathode. Chrom- 
gelatin, tanned by exposure to light, is a suitable positive mem- 
brane. Parchment paper can be used forthe negative membrane. 
If two negative membranes are used, the middle layer becomes 
acid on electrolysis, while with two positive membranes it becomes 
alkaline. This probably depends on the change of concentration 
accompanying flow through membranes as studied by von Bethe 
and Toropoff.2 The anode is a gauze of platinum wire, the 
cathode one of brass wire. The electrolysis is carried out at a 


SENSITIZATION BY MEANS OF HYDROPHILE SOLS 301 


potential of 120 to 240 volts. By use of an additional resistance 
the current is never permitted to exceed 2 amp. This holds the 
temperature below 45°. If the middle layer is kept in constant 
motion it is possible to make the diluted serum (the serum is 
diluted five to ten fold) as free from electrolyte in the course of 
10 to 40 minutes as it could be obtained after a week of analysis. 





Fia. 1.—Apparatus for electrolytic purification of serum. M, Middle cell; 
A, anode space; K, cathode space; 1, chrom-gelatin membrane; 2, parchment 
membrane; 3, platinum gauze; 4, brass gauze; 5, cooling coil; 6, stirring device; 
7, inflow for distilled water; 8, outflow for distilled water. 


The magnitude of the increased sensitivity depends on the 
ratio of albumin to ferric oxide. With increasing ratio of albumin 
to ferric oxide the sol becomes more sensitive. In the following 
paragraphs are given a series of experiments with sols containing 


302 COLLOIDAL BEHAVIOR 


a constant percentage of albumin but with increasing ferric oxide 
content. 

With increasing content of ferric oxide the behavior of the 
mixture approaches that of the pure ferric oxide sol. This is 
shown more distinctly in Fig. 2. The abscissae represent the 
concentration of sodium chloride, the ordinates the degree of 


Degree of Flocculation 





Concentration millimols per liter 


Fig. 2.—The concentration of sodium chloride required to produce floccula- 
tion of an albumin sol sensitized with increasing amounts (I to V) of ferric 
oxide sol. (Cf. Table ITI.) 


coagulation measured in arbitrary units. The Roman numerals 
refer to the corresponding numbers in Table III. 

With high albumin content, the strong peptizing action of 
sodium chloride in high concentration becomes noticeable. 
The peptization is associated with a transfer of electricity, such 
as occurs with globulin suspensions.? In both cases the anion 
concentration which produces the peptization is less the greater 
the valence of the anion and the more readily it is adsorbed. 


SENSITIZATION BY MEANS OF HYDROPHILE SOLS 303 


TasBLeE III 


Albumin sol dialyzed for 7 days. 


Concentration 7.58 g. per liter; 50 cc. 
mixed with increasing amounts of the same ferric oxide sol used in Table I. 
The pure ferric oxide sol used for comparison contained 50 cc. water and 
25 ec. ferric oxide sol. 


























Concen- 50 cc. albumin sol mixed with 
tration 
of NaCl, 
‘a 4 ce. Ce, | 15 ec. 25. 6G; e ee 
1 9 3 a oxide sol 
per liter 5 
1.56| Clear, not 
flocculated 
3.18) ~Turbid Clear, not 
flocculated 
6.25 | Completely | Clear, not | Clear, not 
flocculated | flocculated | flocculated 
12.50 | Completely | Completely | Clear, not | Clear, not 
flocculated | flocculated | flocculated | flocculated 
25.00 | Completely | Completely Turbid Clear, not 
 flocculated | flocculated flocculated 
50.00} Turbid, | Completely | Completely | Turbid Clear, not 
partly flocculated | flocculated flocculated 
flocculated 
100.00} Scarcely Turbid Completely | Completely | Turbid 
turbid flocculated | flocculated 
200.00 | Clear, not Searcely | Completely | Completely | Completely 
flocculated turbid flocculated | floeculated | flocculated 
moe OU reGlear nob 0065... Completely | Completely | Completely 
flocculated flocculated | flocculated | flocculated 











The albumin ferric oxide sol, without addition of electrolyte, 


is not noticeably less stable than the pure sol. 


hydrophile sol alone can coagulate the hydrophobe. 
when a silver sol is mixed with increasing amounts of gelatin.?° 
From the following data it can be seen that very small concen- 
trations of gelatin produce increased sensitivity, while a definite 
larger concentration causes flocculation without any addition of 
electrolyte. 


In some cases the 
This occurs 


3004 COLLOIDAL BEHAVIOR 


TABLE IV.—CoaGuLATION VALUES FOR STRONTIUM NITRATE 


i 











Concentration Concentration of silver sol in grams per liter 
of gelatin, 
milligrams 
per liter 0.375 0.75 1.5 
0 0.55 0.55 0.55 
5 0.46 0.50 
10 0.22 0.42 0.52 
15 Flocculated with- 
out electrolyte 
20 Flocculated with- 0.28 0.46 
out electrolyte 
30 Protected Flocculated with- 
out electrolyte 
40 Protected Floceulated with- 0.23 
out electrolyte 
50 Protected Protected 0.14 
60 Protected Protected Flocculated with- 
out electrolyte 
80 Protected Protected | Flocculated with- 
out electrolyte 
100 Protected Protected Protected 














This table also shows that the behavior of silver sols is similar 
to that of albumin ferric oxide sols, the increased sensitivity being 
the more pronounced the greater the ratio of hydrophile sol to 
hydrophobe sol. The negative silver sol of Carey Lee acts 
like the positive ferric oxide sol. With a certain larger concentra- 
tion of gelatin a protective action appears. The greater the 
concentration of the original hydrophobe sol, the higher the 
concentration of the hydrophile sol at which protective action 
begins. This demonstrates in a striking manner the common 
experience that one and the same hydrophile sol may act as a 
protective colloid in high concentrations, while in low concentra- 
tions it may produce an increase in sensitivity. For this reason 
one cannot properly speak of a substance as a protective colloid. 
It is necessary to specify under what conditions, especially of 
concentration, it exhibits such behavior. 


SENSITIZATION BY MEANS OF HYDROPHILE SOLS 305 


No precise statement can be made of the properties that 
must be possessed by a pair of colloids in order to exert such 
reciprocal influences. Of the pronounced hydrophobe sols, 
Donnau’s acid gold sol! (gold chloride solution reduced by carbon 
monoxide) resembles closely the silver sol of Carey Lee. Ganz?? 
obtained a sudden change of properties by use of gelatin. Freund- 
lich and Loening!® were able to demonstrate exactly, as in the 
case of the silver sol, the same series of phenomena—.e., an 
increased sensitivity with small concentrations of gelatin, 
coagulation by a slightly larger concentration, and with still 
larger concentrations a protective action. 

Even the less distinctly hydrophobic sols as substantive dyes 
and night-blue can be sensitized. Brossa‘* found that a coarse 
suspension of euglobulin can be peptized by these dyestuffs 
and that the clear sol so obtained is much more sensitive to 
electrolytes than the pure dyestuff sols. This fact is brought 
out in the following tables, not only for a negative Congo red sol 
but also for a positive one of night-blue. 


TABLE V 


Concentration of euglobulin 20 g. per liter; 10 cc. mixed with 20 cc. of 
Congo red solution. Concentration of Congo red solution 1 g. per liter. 
The Congo red solution used for comparison was made by mixing 10 cc. 
of water and 20 cc. of Congo red solution. 





pe euereon RE Euglobulin-Congo | Pure Congo 
NaCl, millimols 
: red sol red sol 
per liter | 

5.0 Not flocculated, clear Not flocculated, clear 
12.5 Completely flocculated | Not flocculated, clear 
25.0 Completely flocculated | Not flocculated, clear 
50.0 Completely flocculated | Not flocculated, clear 
100.0 Completely flocculated | Not flocculated, clear 





200.0 Completely flocculated | Not flocculated, clear 


306 COLLOIDAL BEHAVIOR 


TasLe VI 


20 cc. euglobulin solution used in Table IV mixed with 20 cc. night-blue 
solution. Concentration 1 g. per liter. Pure night-blue sol used for 
comparison made by mixing 20 ec. of water and 20 cc. of sol. 





Concentration of 





NaCl, millimols | Mixed sols | Pure night-blue sol 
per liter | 
12 | Turbid Not flocculated, clear 
25 Completely flocculated | Not flocculated, clear 
50 . Completely flocculated | Completely flocculated 
100 Completely flocculated | Completely flocculated 
200 Not flocculated, clear* | Completely flocculated 





* Due to peptization by high concentration of sodium chloride (see Tables I and III). 


Recent work has shown that gelatin is able also to sensitize 
benzoin purple sols.'4 

The previous examples have indicated that gelatin belongs 
distinctly to the class of colloids capable of producing increased 
sensitivity. The same is true of casein. With albumin this 
action is not regularly observed. It does not sensitize the above- 
mentioned dyestuffs. Silver sol is not sensitized by gum arabic, 
saponin, or tannin. On the other hand, tannin is able to sensi- 
tize in a very distinct manner not only positive but also negative 
dye sols,!® the theoretical importance of which fact will be 
developed later. For this reason, the experiments of the 
following tables were carried out with the negative alkali-blue 
sol as well as with that of the positive night-blue. 


SENSITIZATION BY MEANS OF HYDROPHILE SOLS 307 


TaBLE VII 


Concentration of tannin sol 10 g. per liter; 1 ec. mixed with 10 ce. alkali- 
blue sol of a concentration of 1 g. per liter. Pure alkali-blue sol used for 
comparison made by mixing 10 cc. alkali-blue sol with 1 cc. of water. 











Concentration of 
NaCl, millimols Mixed sol | Pure alkali-blue sol 
per liter 
50 Not flocculated, clear 
100 Not flocculated, clear 
250 Completely flocculated 
400 Completely flocculated | Not flocculated, clear 
500 Completely flocculated | Not flocculated, clear 
1,000 Completely flocculated | Completely flocculated 
2,000 Completely flocculated | Completely flocculated 
Tasie VIII 


One and a half cubic centimeters of same tannin sol as used in Table VII 
mixed with 20 cc. of night-blue sol of a concentration of 0.5 g. perliter. 
Pure night-blue sol used for comparison made by mixing 1.5 cc. of water 
and 20 cc. night-blue sol. 


Concentration of 
sodium sulfate, milli- Mixed sols Pure night-blue sol 
mols per liter 








Not flocculated, clear 
Not flocculated, clear 
Completely flocculated | Not flocculated, clear 
Completely flocculated | Not flocculated, clear 
Completely flocculated | Completely flocculated 





Orolo 


re 





It is difficult to explain the important phenomena associated 
with the increase of sensitivity. In all probability it can be 
safely assumed that the two sols form a loose chemical combina- 
tion. This is clearly indicated for the albumin ferric oxide sols.® 
It is also shown by the clear appearance of the flocculate from the 
sol containing albumin. Quantitative studies were made by 
mixing the same amount of albumin sol of known concentration 


308 COLLOIDAL BEHAVIOR 


with different amounts of ferric oxide sol, coagulating with 
sodium chloride, and, after centrifuging, determining the albumin 
content by the Kjeldahl method. Table IX demonstrates that 
a considerable amount of albumin combined with the ferric oxide. 


TaBLE IX 


Twenty-five cubic centimeters of albumin sol of a concentration of 8.04 
g. per liter were mixed with 7.0 cc. of a ferric oxide sol containing 0.5, 1 
2, 5, and 7 cc. of the original ferric oxide sol used in Table 1. ‘The mixed 
sols were coagulated by 5 cc. of a 0.1 normal solution of sodium chloride, and 
the albumin content of the solution determined after removing the flocculate 
by centrifuging. 





Ce. of original Concentration of Grams albumin 
ferric oxide albumin in solution, adsorbed by 1 g. 
sol used grams per liter of ferric oxide 











(tA" 1.25 1.23 
5.0 1.55 1.62 
2.0 2.56 3.12 
1.0 3.47 4.21 
0.5 5.51 3.88 





The amount of combined albumin increases at first in a linear 
manner with its concentration in the solution. At high concentra- 
tions a point of saturation is reached, a fairly constant maximum 
value. A similar behavior is exhibited by other substances of 
high molecular weight towards other adsorbents, as in the 
adsorption of the dyestuff, Ponceau R. R., by barium sulfate.’ 
This is not an adsorption of the simplest type, as it is not reversi- 
ble. The albumin cannot be separated from the ferric oxide by 
washing. It probably undergoes some chemical change on the 
surface of the ferric oxide, perhaps similar to that occurring in 
the process of denaturization.!® It can scarcely be doubted that, 
in the increase of sensitivity produced by gelatin or metal sols 
or by tannin and euglobulin on dyestuff sols, a loose chemical 
combination of the different colloidal particles has occurred. 

On cataphoresis the particles of the albumin ferric oxide 
sol travel much more slowly than those of the pure ferric oxide 


SENSITIZATION BY MEANS OF HYDROPHILE SOLS 309 


sol.6 According to Burton, the particles of the latter move 
4 mm. in 5 minutes under an external potential of 220 volts, 
the electrodes being separated by 16.5 cm. The cataphoretic 
migration velocity » was found to be 0.00106 cm. per second. 
For an albumin ferric oxide sol under identical conditions, the 
migration velocity was 1.5 mm. in 6 minutes, or u equals 0.00042 
cm. per second. wis connected with the so-called electro-kinetic 
potential 2, assumed to exist between the two sides of the elec- 
trical double layer of the particles by the equation 


where 7 is the viscosity of the liquid, H the external potential in 
volts per centimeter, and D the dielectric constant. For the pure 
ferric oxide sol, = is equal to 12 millivolts, while for a sol contain- 
ing albumin the value was 4.8 millivolts. In the same manner 
gelatin depresses the cataphoretic migration velocity of the 
particles of a Carey Lee silver sol (see Table X). 

The above facts lead to the following conception of the increase 
in sensitivity. Solutions of gelatin, casein, and albumin are 
sols which may be assumed to contain the so-called colloidal 
ions in large amounts. ‘The colloidal particles of these sols have 
comparatively small atomic weights, probably of the magnitude 
10,000 to 100,000. These particles can be ionized and carry 
a correspondingly large number of charges. The total charge is 
large compared with that on a simple ion as Kt, but much smaller 
than the total charge carried by a gold particle in a gold 
sol, as the latter probably holds thousands of unit negative 
charges. 

A correspondingly large number of cations from the solution 
form the opposite side of the electrical double layer. One can 
scarcely speak of a double layer, however, in connection with 
colloidal ions. The lines of force between oppositely charged 
ions run irregularly as with the ordinary electrolytic solution. 
They are not grouped around a certain middle point, as with the 
coarser particles of a gold sol. The characteristic behavior of 
such colloidal ions in soap solution has been studied by McBain.” 
Because of the amphoteric nature of protein solutions colloidal 
cations as well as colloidal anions are present, the former in acid 
solution, the latter in alkaline solution. 


310 COLLOIDAL BEHAVIOR 


It can be safely assumed that, for the hydrophobe sols to which 
ferric oxide sol belongs by virtue of its reaction towards electro- 
lytes, coagulation depends upon a reduction of the 2 potential 
of the particles accompanied by a decrease in their charge. 
Below a certain > value, particles coming in contact adhere to 
each other. The ion carrying a charge opposite to that of 
the micelle is responsible for the reduction of the 2 poten- 
tial. It is probable that the colloidal ions can in some respects 
act like inorganic ions. They can reduce the 2 potential, lower 
the stability of hydrophobe sols, and even produce coagulation 
(see Table IV). Or they can bring the sol into a condition where 
the particles are partly discharged, when only a small concen- 
tration of electrolyte becomes necessary for coagulation. 

In an approximately neutral albumin solution, only the pres- 
ence of enough anions to discharge and sensitize the positive 
micelles of the ferric oxide sol need be assumed. In a gelatin 
solution enough cations must be assumed to be present to do the 
same for a negative silver sol or for an acid gold sol. 

Completely in accord with this point of view is the fact that a 
gelatin solution does not sensitize a gold sol prepared by reduc- 
tion with formaldehyde.!®° This sol is distinctly alkaline and 
the gelatin solution may not contain a sufficient number of 
cations. 7 

Any increase in sensitivity produced by the cations can only 
appear when they are present in a low ratio to the micelle of the 
hydrophobe sol. When the particles of the hydrophobe are 
surrounded by larger amounts, they necessarily assume the 
properties of the hydrophile colloid. The protective action 
appears as soon as the concentration of hydrophile micelle is 
sufficiently large (see Table IV). The nature of the charge 
carried by the particles in this region is not definitely known. 
From Table IV, in the case of gelatin-silver sol, one would expect 
the particles to be positively charged, due to the discharge 
of negative silver particles by cations. The protective action 
would then be exerted by a layer of cations. But this is not 
the case. The gelatin-silver sol in the region of protective action 
is negatively charged! (see Table X). 

This is understood after considering the amphoteric nature of 
the proteins. It appears possible that the gelatin micelles, 


SENSITIZATION BY MEANS OF HYDROPHILE SOLS 311 


which cover the negative silver particles, lie with their positive 
end towards the silver particles while their negative end projects 
into the solution. The particles as a whole would then be nega- 
tively charged, which is observed to be the case. This concep- 
tion has been developed from the theory of Langmuir!® and 
Harkins.1® According to this theory, the COOH group of the 
fatty acid is turned towards the water, leaving the hydrophobic 
end of the molecule to form the surface. 


TABLE X 
Concentration of gelatin, : 
nite : uw in centimeters per second 
milligrams per liter 
0 —0.0016 
5 —0.0014 
mel —0.0009 
20 —0.0008 
30 —0.0007 flocculation begins 
65 —Q.0008 protected 
70 —0.0011 protected 
80 —0.0011 protected 
100 —0.0011 protected 
200 . —0.0011 protected 





While this conception of colloidal ions explains in a satis- 
factory manner the properties of albumin ferric oxide sol, gelatin- 
silver sol, etc., it may not be generally valid. It does not explain 
the increased sensitivity of dyestuff sols (both acid and basic) 
croduced by tannin (see Table X). Tannin is only slightly 
pissociated in solution, is not amphoteric, and can scarcely be 
donceived of as producing colloidal ions. On cataphoresis the 
migration velocity of the positive night-blue sol is probably 
reduced, while that of the negative alkali-blue sol is not appre- 
ciably changed. Nevertheless, both are sensitized to about the 
same extent (see Tables VII and VIII). 

Tests were made to see if a change in hydrogen ion concen- 
tration due to the added tannin was responsible for the increased 
sensitivity. But tannin also increases the sensitivity of pure 


312 COLLOIDAL BEHAVIOR 


hydrophile sols and renders them more easily coagulated by elec- 
trolytes. Kruyt?° has explained its action on agar sols by 
assumptions based on the Langmuir-Harkins theory. The tannin 
molecule can be considered as polar. The glucose part of the 
molecule is pronouncedly hydrophilic, the digalloyl part is hydro- 
phobic. It is probable that when the tannin particles come in 
contact with the agar micelles they become orientated so that the 
hydrophobic end projects into the solution. The new complex 
micelles are then as a whole hydrophobic the same as the tannin 
and agar micelles alone. This conception can also be employed 
for the tannin dyestuff sols. The dyestuff sols are certainly less 
hydrophobic than the metal or even the hydroxide sols. They 
are markedly more sensitive towards monovalent inorganic ions. 
It is conceivable that their micelles orientate themselves in a 
manner similar to that assumed for the agar micelle. The tannin 
dyestuff micelle as a whole would then be hydrophobic as are the 
tannin and dyestuff micelles alone. 

Apparently the same conceptions that were forced upon us in 
the case of sensitization through tannin must be employed in 
the cases that were first explained from the point of view of 
colloidal ions, for example, the sensitization of dyestuff sols by 
euglobulin (see Tables VI and VII). Euglobulin, being a pro- 
tein, is amphoteric and would be expected to sensitize positive 
as well as negative sols. But it is surprising to find that it exerts 
only a protective action on dyestuff sols and does not sensitize 
them. Furthermore, the influence of alkali is not that which 
we would expect from the action of colloidal ions on coagulation. 
It seems necessary to explain the behavior on other grounds than 
that of the amphoteric nature of the protein, and its ability to 
form two types of colloidal ions. There exists, perhaps, a polar 
orientation such as was assumed for tannin. 

It is scarcely necessary to state that the explanation given for 
tannin represents only one of the possibilities. Some special 
form of chemical combination with the dyestuff is conceivable, 
or the factors influencing the increased sensitivity of hydrophobe 
sols by non-colloidal non-electrolytes may have to be considered. 
Sols may be sensitized by substances that are not colloids, as 
ferric oxide sol by camphor thymol, and other substances. It 
has been assumed hitherto that this is due to a change in the 


SENSITIZATION BY MEANS OF HYDROPHILE SOLS 313 


dielectric constant at the boundary surface, which influenced the 
charge and therewith the stability of the particles. ?! 

In any case, we have the following choice in dealing with this 
problem. If we desire a general solution, the influence of colloi- 
dal ions is certainly not sufficient, and it is necessary to test the 
hypothesis employed in the case of tannin to determine if it is 
generally valid. Or we may abandon the hope of a general 
solution and assume that the increased sensitivity is due to 
different factors in different cases. The conception of colloidal 
ions is completely satisfactory in many cases, but for others it 
is necessary to consider special possibilities, such as a polar struc- 
ture in the case of tannin. It seems very probable that the 
phenomenon has no one special cause but must be referred to 
several factors. 


USE OF THE PROCESS 


Although the above explanation of the increased sensitivity of 
sols leaves much to be desired, it is useful in the understanding of 
biological processes and in the solution of technical problems. 
Its usefulness depends upon the fact that the hydrophile colloids 
differ fundamentally in their ability to produce an increased 
sensitivity. These differences can be used as a means of identifi- 
cation. This is important because other convenient methods are 
often inadequate. This principle can be made use of in precipi- 
tating hydrophile colloids otherwise very difficult to separate 
from solution. The sol is mixed with a suitable hydrophobe sol, 
and the sensitized mixture precipitated by electrolytes. 


THE RECOGNITION OF HYDROPHILE SOLS 


By use of this process, Windisch and Bermann?”? could, with a 
high degree of probability, separate the colloids responsible for 
the essential properties of beer foam. The importance of 
colloids in the production of foam is well known, for filtration 
strongly reduces the ability of a solution tofoam. It is necessary 
to distinguish between the colloids which produce foam and those 
whose chief function is to stabilize the foam. On filtering the 
wort through one of de Haen’s membrane filters (No. 64) the 
filtrate still foams after vigorous shaking. This disappears, 


314 COLLOIDAL BEHAVIOR 


however, in 70 to 80 seconds, while with unfiltered wort it persists 
for about 30 minutes. Part of the colloids left on the filter may 
be peptized by water. On adding this sol to the filtrate, the 
mixture recovers to a large degree its ability to produce a perma- 
nent froth. It is, therefore, the material retained on the filter 
and susceptible to peptization by water that imparts the com- 
parative permanence to the foam. 

It is desirable to determine the nature of these substances more 
closely. Chemically, they are not well characterized, but it 
happens that they differ markedly in their behavior towards 
ferric oxide sol. Windisch and Bermann named the concentra- 
tion of sodium chloride necessary for coagulation of a given 
amount of ferric oxide sol, after adding a certain colloid, the 
iron number. This number alone suffices to characterize the 
hydrophile colloids if they are added to the same concentration 
of ferric oxide sol. This, however, was not possible, as the 
differences between the various colloids proved to be too large. 
A concentration of the hydrophile sol small enough to be suitable 
for gum arabic (a strongly sensitizing colloid) would render a 
weaker sensitizing colloid, as gelatin, inactive. A concentration 
large enough for gelatin was far too large for gum arabic. The 
gum arabic would be coagulated even without the addition of 
electrolyte. It is necessary, therefore, to use different concen- 
trations of the hydrophile colloids. Windisch and Bermann 
discuss the iron number and the order of magnitude of the various 
colloids. Their data are given in Table XI. 

















_ TABLE XI 
: | Order of Tron 
Hydrophile colloid | enemies eee 
Gum arabie. so. or. ae ee 0.01 9.4 
Agar ia dics M00 ee 0.01 9.4 
Gelatinecas fo ad. oy inated ee 0.05 9.4 
Witt’s ‘peptone... 1.1099. 2 eae ee 0.10 9.4 
Albuminoses (from wort);<.9 62.95 eee 0.10 9.4 
Barley gum. 25.004. fol) eee 0.10 4.7 
Dextrin’, of 5 eee ee Acts only 
protectively 


| | 


Meher. 


SENSITIZATION BY MEANS OF HYDROPHILE SOLS 315 


Only the iron number and order of magnitude of the colloids 
(separated by the No. 64 ultra filter and then peptized) were 
determined. They found an order of magnitude of 0.1 and an 
iron number of 4.7, which correspond well with those of barley 
gum (Table XI). Barley gum, albuminoses, and peptins can be 
detected in the peptized colloids. A mixture of barley gum 
and Witt’s peptone gave similar values. As it has been shown 
that the albuminoses are probably, above all colloids, responsible 
for the existence of the foam, we may conclude that barley 
gum produces the permanence of the foam 

Stimulated by this research, Reitstétter?? studied the iron 
number of different protein fractions of normal and pathological 
blood serums. For purification and separation of the different 
fractions, he used the above-described electrolytic method. 
On electrolyzing the serum in the middle chamber of the appara- 
tus, the euglobulins separate out after the action has progressed 
sufficiently. These are then centrifuged off and a solution of 
albumins and paraglobulins obtained. By “paraglobulins”’ is 
meant the protein fractions lying between albumins and euglobu- 
lins. They remain behind on dissolving the albumins in pure 
water. They can be salted out in the same manner as euglobu- 
lins by half saturation of the solution with ammonium sulfate. 
Paraglobulins and albumins are, therefore, separated by half 
saturation of the solution with ammonium sulfate and filtering 
off the paraglobulins. The filtered material is then peptized 
by water and purified from salts by the electrolytic process 
previously described. 

Reitstétter tested solutions of albumin and paraglobulin 
prepared in this manner for their action on ferric oxide sol. 
The determination of the iron number was sufficient for identifica- 
tion, since the differences between the protein fractions were not 
large enough to necessitate a determination of the concentration 
order. The measurements were made in exactly the same 
manner as those of Freundlich and Brossa. !: 2}3}5 Not over 2 ec. 
of a 0.649 per cent ferric oxide sol were added to 25 cc. of a pro- 
tein solution of about 0.6 per cent concentration. After adding 
5 ec. of the mixture to 1 cc. of electrolyte solution the coagulation 
was measured. 


316 COLLOIDAL BEHAVIOR 


Taste XII 


ee 


Coagulation value in millimols per liter 


Protein hydrosol 











Sodium | Barium | Sodium | Potassium 
chloride | chloride | salicylate| sulfate 
Protein-free ferric oxide sol..... .| 87.500 | 37.500 | 2.300 2.300 
Albumins from human _ blood 
BOLUM oat «bitin piace ee ee 0.290 | 0.290 | 0.097 0.097 
Cattle albumins................) 0.23854" Oi) 235s gees 0.097 
Horse albumins?.....c.0-) eae 0.285.| 0.235 0.097 0.097 
Diphtheria albumins. . 0.235 |) 0-235 02007 0.097 
Paraglobulins from haman ier 
SOLU Shc halek hw & ee ee 4.690 | 4.690 lel 70 1.170 
Cattle paraglobulins............ 4.690 | 4.690 | 1.170 1.170 
Horse paraglobulins............ 4.690 | 4.690 1.170 1.170 
Dysentery paraglobulins........ 2.340 | 2.350 | 0.586 0.290 
Diphtheria paraglobulins........ 2.340 | 2.350 | 0.586 0.290 
Tetanus paraglobulins.......... 1.170 | 2.850 | 0.097 0.097 
Chicken cholera paraglobulins...| 9.370 | 4.690 | 2.350 Lig 
Hog erysipelas paraglobulins....| 4.690 4.690 1.170 1.140 








Although the albumins of healthy and pathological serum 
reacted the same, the paraglobulins were in some cases distinctly 
different. The paraglobulins of dysentery, diphtheria, and 
tetanus sensitized much more strongly than those from normal 
serum; the chicken cholera paraglobulins sensitized more weakly. 
The paraglobulins of hog erysipelas reacted like those from normal 
serum. This is probably the first time that it has been possible 
to identify in a test tube, by means of colloid chemistry, the 
proteins of normal and pathological serum. This becomes 
important, since it has been shown that the gold number cannot 
be used as a means of differentiation.2* The albumins do not 
exert so strong a protective action as the paraglobulins. How- 
ever, the values for the albumins and paraglobulins le too close 
together to serve as a characteristic means of identification. 

Reitstétter did not study euglobulin in this manner. The 
usual methods could not be employed, as this protein precipitates 
in pure water. It must be taken up by a salt solution whose 


rays 


eT 


SENSITIZATION BY MEANS OF HYDROPHILE SOLS 317 


concentration alone is great enough to coagulate the ferric 
oxide sol. Brossa!* overcame this difficulty by using dyestuff 
sols in place of ferric oxide sols. It has already been stated that 
such sols are capable of peptizing coarse suspensions of euglobulin. 
The sol so obtained is strongly sensitized. A comparison between 
the euglobulins from different serums has not yet been made but, 
as was briefly mentioned above, a striking difference has been 
demonstrated between albumins and euglobulins, since the former 
exerts a protective action and the latter renders the sol more 
sensitive. Even in very low concentration, the albumins do not 
sensitize. Table XIII shows that albumins protect not only 
Congo red sols but also euglobulin Congo red sols. 


TaBLe XIII 
Concentration of euglobulin suspension 40 g. per liter; 50 cc. mixed with 
50 cc. of Congo red solution containing 1 g. per liter. The albumin solution 
was prepared by electrolysis and contained 10 g. per liter. 


Ce. albumin Cc. water | Effect on mixed 
solution added added sols 








Completely flocculated 
Completely flocculated 
Completely flocculated 
Clear, not flocculated 
Clear, not flocculated 





Ore 
So 00 © © 


In this research 1 ce. of albumin solution of different concen- 
trations was added to 1 ce. of the mixed sols, and 0.5 ce. of normal 
sodium chloride solution added. 

We may conclude that, in general, or at least towards dyestuff 
sols, euglobulin and albumin sols react in an opposite manner. 
Brossa established this for a number of cases. If then, in a 
serum, the concentration of albumin or of euglobulin is artificially 
increased, the new serum reacts differently towards a dyestuff 
sol. In order to increase the content of euglobulin, some of the 
protein prepared as described above is dissolved in a so-called 
normal solution (a salt solution of the same composition as that 
of the salts in the serum) and then added to the serum. ‘To 
increase the ratio of albumin to paraglobulin, use is made of the 


318 COLLOIDAL BEHAVIOR 


solution obtained by coagulating the euglobulins by electrolytes. 
It has been shown that the serums of different animal species 
of nearly equal protein content, but having different ratios of 
euglobulin to albumin, sensitize Congo red more strongly the 
greater the amount of euglobulin present. The ratio of euglobu- 
lin to albumin in the following serums is: 


Rabbit serum. ocr cis «soles ae alk '= cone one been ate ehh 
Phrman Serum... os. sc vceip wes cts od 0 ns eee LoL 
Horse SCIUM =). os oe eS cee 's we bv ee 0 an ne 1:0.58 


Thus, a mixture of Congo red and rabbit serum is only coagulated 
by a high concentration of electrolyte, while a mixture of horse 
serum and Congo red is affected at low concentrations. In 
many pathological conditions this ratio of euglobulin to albumin 
is decidedly changed. 

For healthy rabbit serum the ratio varies from 1: 2.36 to 1: 3.59. 
It changes to 1: 1.52 for a typhus immune serum. The immune 
serum, accordingly, sensitizes Congo red much more than the 
normal. The same can be demonstrated for the serum of men 
in normal health and men sick with typhus. 

This research has lead to a distinction between euglobulins 
and albumins which is, perhaps, of general significance. Accord- 
ing to recent work of von R. Stern,” the mechanism of the 
Wassermann and Sachs-Georgi reactions exhibit similar relation- 
ships. He was able to point out that a positive reaction in syphi- 
litic serum was connected with the euglobulin fraction and not 
with the albumin of paraglobulin fractions. When the proteins 
of the serum are separated from one another in the manner 
described above, only the euglobulin and not the paraglobulin or 
albumin fractions gave a positive reaction in the Wassermann and 
Sachs-Georgi tests. In contrast with the results given in Table 
XII, only the euglobulin and not the paraglobulin fraction of 
syphilitic serum is changed. To speak more accurately, we 
must distinguish two parts in the euglobulin fraction. Only 
that fraction is active which is obtained by a complete purifica- 
tion from electrolytes. ‘The labile euglobulin obtained by the 
action of carbon dioxide or by dilution of the serum is inactive. 
The change in the syphilitic euglobulins cannot of course be 
shown by a changed sensitivity of the dyestuff sol. It is possible 


pec ct 


SENSITIZATION BY MEANS OF HYDROPHILE SOLS 319 


that the dyestuff sols are not sufficiently sensitive, while the 
colloidal solutions of the extracted lipoids are. If in the Sachs- 
Georgi reaction the lipoids extracted from syphilitic serum or 
from syphilitic euglobulin are more easily coagulated by salts 
than those from healthy serum or euglobulin itself, then the 
reaction consists in nothing more than a sensitization. The 
Wassermann reaction is intimately connected with that of Sachs- 
Georgi. The greater ease of flocculation in the Sachs-Georgi 
reaction corresponds to a stronger adsorption capacity for the 
substances that produce the Wassermann reaction. 

This idea becomes more plausible when we recall that, according 
to Stern, the syphilitic euglobulins can be replaced by a colloid 
such as tannin, which acts in a manner similar to that of euglobulin 
in increasing the sensitivity of dyestuff sols. On mixing a healthy 
or Wassermann negative serum with tannin, a positive Wasser- 
mann or Sachs-Georgi test is obtained. However, it is not neces- 
sary to add the tannin totheserum. Thesolution of the extracted 
lipoids is sensitized by tannin, and becomes more easily coagulated 
by electrolytes. It was distinctly indicated that the effect is 
due to the specific nature of tannin, and is not a hydrogen ion 
effect of the weakly acid tannin solution. In a buffer solution of 
pH 7.0 to 7.6 the same result was obtained. According to our 
theory, this result can be explained as follows: The syphilitic 
euglobulin is distinctly more polar than the normal euglobulin 
and thereby makes the mixture of extracted colloid and euglobu- 
lin a hydrophobe sol in the same manner as was described for 
tannin. It should be especially emphasized that no change of 
charge can be detected either in the syphilitic euglobulin or in the 
syphilitic serum. Therefore, no explanation based on colloidal 
ions can be valid. 

The phenomenon of agglutination can be conceived of as 
probably due to an increased sensitivity. It consists essentially 
in an adsorption by the bacteria of agglutinating substances, pro- 
duced by their life process. They are thus changed by agglu- 
tinating bacteria to suspensions which are more easily precipitated 
by electrolytes than the original bacterial suspensions. From 
what little is known of the nature of agglutinin, it seems probable 
that it is a hydrophile colloid. The original bacterial suspension 
cannot, of course, be classed as a hydrophobe. It is not sensitive 


320 COLLOIDAL BEHAVIOR 


to the effects of the cations of alkali or alkaline earth metals, 
while the suspension of the agglutinating bacteria has a coagula- 
tion value corresponding to that of a hydrophobe sol. Moreover, 
a typical hydrophile suspension is changed to a hydrophobe by 
agglutination. But it is an unsolved problem as to whether a 
sol must be a hydrophobe in order to be sensitized. Perhaps one 
hydrophile can sensitize a second one by changing its micelle into 
hydrophobic micelle. The dyestuff sols which have been dis- 
cussed are so little affected by alkali salts that they cannot be 
classed as true hydrophobes. Kruyt has shown that the markedly 
hydrophile agar sol is changed to a hydrophobe by tannin. If 
this increased sensitivity is due to the same cause previously 
assumed to explain the action of tannin (the adsorption of a polar 
colloid accompanied by an orientation of this colloid so that the 
hydrophobe group projects into the solution), then it is possible 
that the same effect is produced by the adsorption of polar colloids 
by hydrophile micelle. There would then be no hesitation over 
classifying the behavior of tannin agar sols together with the 
agglutinating bacteria as a sensitization effect. 

It is to be noted that agglutination is distinctly specific. 
Typhus bacteria, for example, are affected only by an agglutinin 
produced by inoculating an animal with typhus bacteria, and 
are not acted on by agglutinins obtained from other organisms. 
One recalls that according to Reitstétter’s work, healthy and 
pathological paraglobulins, which in other respects are chemi- 
cally and physically identical, can be distinguished through their 
different sensitization effects. It may be that sensitization is in 
many cases extremely delicate, and that slight differences may 
strongly depress the ability of colloids to sensitize. This leads 
apparently to the decided specificity of agglutination. Possibly 
bacteria and agglutinins must be adjusted to each other in a 
definite manner. Only then will the hydrophile groups of the 
agglutinin be oriented towards the solution and an increase in 
sensitivity be observed. If this adjustment does not occur at 
all, or is only imperfect, then no sensitization takes place. 


Tut REMOVAL OF PROTEINS 


The so-called deproteinization by ferric oxide sol, which was 
introduced by Michaelis and Rona,*® depends upon sensitization. 


SENSITIZATION BY MEANS OF HYDROPHILE SOLS 321 


A frequent problem in physiological work is that of removing 
protein from a liquid and at the same time changing the liquid as 
little as possible. In order to deproteinize serum, only 50 cc., 
diluted 12 to 14 times, is added drop by drop with constant 
shaking to 40 cc. of an electrolyte-free ferric oxide sol of suitable 
concentration. The amount of ferric oxide sol used can be 
adjusted so that the flocculate contains all the protein as well as 
all the iron. ‘The filtrate is completely free from both. Since 
an amphoteric protein sensitizes positive as well as negative sols, 
it would seem possible to use a mastic sol in place of the ferric 
oxide sol. But this requires an addition of electrolyte for com- 
plete coagulation. 

The removal of protein depends upon the same causes that are 
active in sensitization. The complex of ferric oxide and albumin 
micelle is coagulated by the salts present in the serum. But 
sensitization differs in one not unimportant respect from depro- 
teinization. In sensitization the substances are mixed in the 
following order: (albumin sol + ferric oxide sol) + electrolyte 
solution. For the process of deproteinization the order is: 
(albumin sol + electrolyte) + ferric oxide sol. 

In sensitization a mixed sol is prepared which is as free from 
electrolyte as possible. In deproteinization the mixed sol used in 
sensitization is of a decidedly amicronic nature. On addition 
of electrolyte it becomes gradually turbid and only precipitates 
after some hours. But in deproteinization, the flocculate 
separates quantitatively after some minutes. This is because 
the electrolyte present prevents the formation of amicronic 
particles. Consequently, large particles form rapidly and are 
soon coagulated. Contact probably plays a part here. Accord- 
ing to Smoluchowski, contact is of great importance in acceler- 
ating coagulation when the colloidal particles exceed a certain 
size. In deproteinization the conditions are favorable for this 
contact influence. In sensitization they are unfavorable. The 
amounts of protein adsorbed in the two cases are not essentially 
different, as can be seen from Table IX. 


CONCLUSIONS 


Sensitization is experimentally a well-defined phenomenon 
of frequent occurrence. It consists in the action of a hydrophile 


322 COLLOIDAL BEHAVIOR 


sol on a hydrophobe (perhaps on a second hydrophile sol), as a 
result of which the sol becomes more easily affected by electro- 
lytes. It is improbable that any single explanation suffices to 
account for this behavior. At least, it cannot always be con- 
sidered as due to the discharge of the micelles of a hydrophobe sol 
by colloidal ions. Thus the action of non-amphoteric tannin in 
sensitizing both positive and negative dyestuff sols and the 
occasional distinct differences in the ability of albumin and 
euglobulin to sensitize are difficult to explain on the hypothesis 
of colloidal ions. Apparently, it is necessary to take into 
account the polar nature of the sensitizing colloids, which enables 
them to orientate themselves so that the complex micelles have 
‘a more pronounced hydrophobic nature than those of the original 
hydrophobe sol. Sensitization is of general importance as a 
method of identifying hydrophile sols. It might be a profitable 
procedure to make use of sensitization methods in all problems 
where it is necessary to identify hydrophile sols. 


REFERENCES 


_ Zstamonpy: “Kolloidchemie,” 3rd. ed., 1920, p. 149. 
. Henri, Latov, Meyer and Stoei: Compt. rend., 56 (1903), 1671. 
_ Nezsser and FrrepEMANN: Muiinch. med. Wochschr., 61 (1903), No. Ls 
. Pauut and Fiecxer: Biochem Z., 41 (1912), 470. 
_ Brossa and Freunpuicu: Z. physik. Chem., 89 (1915), 306. 
For an account of the coagulation of hydrophobe sols, see FREUNDLICH, 
‘‘Kapillarchemie,” 2nd ed., 1920, p. 569. 
7. Ruppey: Ber. pharm. Ges., 30 (1920), 314; RerrsTéTrerR and OESTER: 
Chem. Ztg. (1922), No. 5. 
8. Berne and Toroporr: Z. physik. Chem., 88 (1914), 688; 89 (1915), 597. 
9. Cuick: Biochem, Z., T (1913), 318. 
10. FREUNDLICH and Lomnine: Kolloidchem. Beihefte, 16 (1922), 1. 
11. Donavu: Monatshefte, 26 (1905), 525. , 
12. Ganz: Kolloidchem. Beihefte, 8 (1915), 251. 
13. Brossa: Kolloid Z., 32 (1923), 107. 
14. FreuNDLIcH, ScuustER, and ZocueEr: Z. physik. Chem., 76 (1911), 710. 
15. Mare: Z. physik. Chem., 81 (1913), 641. 
16. Bixtz: Biochem Z., 23 (1909), 27. 
17. For a collected account of the researches of McBain and his co-workers, 
see Freunpuicuy, “Kapillarchemie,” 2nd ed., 1922, p. 774. Cf. Chap. 
XVI. 
18. Lancmuir: J. Am. Chem. Soc., 38 (1916), 2221; 39 (1917), 1848. 


OAoPrwndr 


oe 


19. 


20. 
21. 
22. 
23. 
24. 
25. 


26. 


27. 


SENSITIZATION BY MEANS OF HYDROPHILE SOLS 323 


Harkins, Brown, and Daviss: J. Am. Chem. Soc., 39 (1917), 354. 
Harkins, Davies and Cuark: tbid., 39 (1917), 541. Harkins, CLARK, 
and Roperts: ibid., 42 (1920), 700. Cf. Chap. VI. 

Kruyt: Kolloid Z., 31 (1922), 338. 

FREUNDLICH and Rona: Biochem Z., 81 (1917), 87. 

WInpIscH and BERMANN: Wochschr. fiir Brauerei, 37 (1920), 130. 

REITsTOTTER: Z. Immunitdt, 30 (1920), 507. 

REITSTOTTER: [bid., 30 (1920), 486. 

Stern, R.: Klinische Wochenschrift, 2 (1923), 145, and a later number 
yet to be published. 

MicHak.is and Rona: Biochem Z., 2 (1907), 219; 3 (1907), 109; 4 (1907), 
11; 5 (1907), 525; 7 (1908), 329; 8 (1908), 356; 14 (1908), 476. Rona 
and OppiER: [bid., 13 (1908), 121. 

v. SMOLUCHOWSKI: Z. physik. Chem., 92 (1917), 155. 


CHAPTER XIII 
MUTUAL REACTIONS OF COLLOIDS 


By 


ARTHUR W. THOMAS 


When colloidal solutions are mixed, decrease in stability 
leading to precipitation may result, or, if one of the components 
of the mixture is a very stable colloid (hydrophilic) and is added 
in excess, while the other is rather unstable (hydrophobic), no 
visible change may take place and the solution will be found to 
show the stability and properties of the hydrophilic colloid. 

The latter phenomenon is called “protective action.” The 
former case, mutual precipitation of colloids, will be considered 
first. 


MutTuAL PRECIPITATION 


The phenomenon of mutual precipitation of certain colloids 
was noted by Thomas Graham.! Linder and Picton? showed 
that mutually precipitating sols migrated oppositely in an electri- 
cal field. This was confirmed by Lottermoser,’* who tried also 
to analyze the mixed gels resulting from mutual precipitation, 
but found quantitative results impossible of attainment, due to 
adsorption, etc. It was concluded then that oppositely charged 
sols precipitate each other. Bechhold,* Neisser and Friedman,* 
Henri® and his co-workers, and Teague and Buxton’ published 


1 J. Chem. Soc., 15 (1862), 246. 

2 J. Chem. Soc., 71 (1897), 586. 

3“ Anorganische Kolloide,” Stuttgart, 1901, p. 77 (through ZsiemMonpy- 
Sprar: “Chemistry of Colloids,” John Wiley & Sons, New York, 1917, 
p. 56. 

4Z. physik. Chem., 48 (1904), 385. 

5 Miinch. Med. Wochschr., 51 (1904), 465, 827. 

6 Compt. rend. soc. biol., 55 (1903), 1666. 

7Z. physik. Chem., 60 (1907), 489. 

324 


(On Me Daath, > 


MUTUAL REACTIONS OF COLLOIDS 325 


work confirming this rule. Spring’ stated that upon mixing 
aniline blue (which migrates to the anode) and Magdala red 
(which migrates to the cathode), no precipitation occurred, which 
he thought disproved the work of Linder and Picton and Lotter- 
moser. Biltz® made a quantitative study of mutual precipitation 
by mixing measured quantities of sols which had been previously 
analyzed for the dispersed phases. 

The following table is typical of the many combinations which 
he reported: 

















TaBLe | 
2 ce. sol 13 ce. sol | Appearance 
containing, | containing, 
grams grams 
Sb.S; FeO; Immediately After 1 hour 
5.6 20.8 Turbid Slight precipitation 
5.6 12.8 Turbid Slight precipitation 
5.6 8.0 Slow settling Complete precipitation 
5.6 6.4 Complete precipitation | Complete precipitation 
5.6 4.8 Incomplete precipita- | Incomplete precipita- 
tion tion 

5.6 | 3.2 Slight precipitation Unchanged 


5.6 . 0.8 Slight precipitation Unchanged 
| | 


The ratio between precipitating sols varied so much that the 
only conclusions to be drawn were that there are certain propor- 
tions in which sols must be mixed for complete precipitation; 
outside of this range there is partial precipitation within certain 
limits, and, if either sol is present in great excess, no precipitation 
occurs. Spring’s failure to obtain precipitation with the dyes 
of opposite migration in an electrical field was due probably, to 
the proportions in which they were mixed, or to the fact that 
they do not react to form an insoluble compound. 

In a comparison of precipitating values of the same sol against 
other sols, Biltz demonstrated that, while the optimum amount of 
a positive sol required to precipitate negative sols varies, the order 
is always the same, as shown below: 


8 Bulletin de V Acad. Roy. de Belg. (Sciences) (1900), p. 483. 
9 Ber., 37 (1904), 1095. 


326 COLLOIDAL BEHAVIOR 
TaBLeE II 


Nek Milligrams of positive sol 
Milligrams of 




















negative sol | | 
Fe.03 ThO, CeO, ZrO, Cr.03 | Al,O; 
Au 14 3 2.5 4 130 0.3 Osta 2 
As2S3 24 13 6.0 4 2.0 0.5 2.0 
Sb.S; 32 32 | ai) ae 11 Vera 3 ea 
| | | | 





It was thus obvious that an equivalence between the optimum 
amounts of sols for precipitation exists, but not an equivalence 
between the particles of the dispersed phases of the sols. Billit- 
zer!? suggested that the equivalence is electrical, since at maxi- 
mum precipitation there is no migration in an electrical field, 
and on either side the migration is in the direction of the sol 
in excess. He thought the equivalence would not be exact 
because of variations in the size and numbers of particles and in 
the concentration of the sols. 

The following table is typical of his results: 


Taste III.—Arsenious SuntripE Sot Mrxep wits Ferric Oxipre Sou 
The As2S; sol contained 2.07 mg. As2S3 per cubic centimeter, and the Fe.O; 
sol, 3.036 mg. Fe:O; per cubic centimeter 

















Cubic centimeters in Migration in 
10 ce. of mixture electrical field 
Appearance Ai 
of unprecipi- 
Fe.0; sol | As2S; sol tated portion 
9.0 1.0 No change To cathode 
8.0 2.0 Slight turbidity To cathode 
(ae) on0 Immediate turbidity, then precipi- | To cathode 
tation 
5.0 5.0 Immediate precipitation To cathode 
3.0 7.0 Nearly complete precipitation 
2.0 8.0 Immediate precipitation To anode 
1.0 9.0 Immediate precipitation To anode 
0.2 FLO TS Turbidity To anode 





10° ce. of; 2 | 
times diluted } 10 


Clear To anode 
solution 


10Z, physik. Chem., 51 (1905), 129. 


MUTUAL REACTIONS OF COLLOIDS Oot 


Billitzer also noted that when two colloids of like charge are 
mixed, the less stable one becomes more stable, assuming the 
properties of the more stable sol. 

It is noted that, in the presence of a very large excess of one 
sol, no change in appearance of the system takes place. The 
lack of precipitation in such cases is ascribed to the fact that, 
while a neutralization of charges takes place, flocculation of the 
particles does not ensue, due to the fact that they are adsorbed 
or protected by the colloid in excess. That mixture which results 
in complete precipitation is frequently called the ‘‘isoelectric”’ 
mixture, since there is no migration in an electrical field after 
such an instance, for the obvious reason that nothing is left in 
suspension. 

The theory of mutual precipitation, which arose from the fore- 
going experiments and from the opinion held by some that colloid 
stability resides in the mutual repulsion of like-charged particles, 
is that when oppositely charged colloidal particles are brought 
together an electrical neutralization ensues resulting in agglom- 
eration of the particles; there being no electrical repulsive forces 
left, the particles must settle out of solution. 

In 1910, Lottermoser!! suggested that the equivalence may be 
that of the small amounts of stabilizing electrolyte in the sol, and 
that the precipitation may be due to a chemical reaction between 
the peptizing agents, by which they are removed. To test this 
he mixed positive silver iodide sols stabilized by silver nitrate 
and negative silver iodide sols stabilized by potassium iodide. 
The results summarized in Table IV indicate that the explana- 
tion holds for this precipitation at least. The quantities given 
are millimols at maximum precipitation. 

These experiments, which point to a simple chemical reaction 
between the stabilizing electrolytes of the colloidal complexes 
in precipitation, have not attracted any attention. 

Freundlich and Nathansohn!? have recently shown that the 
mixing of pairs of certain like-charged colloids may result in 
mutual precipitation. For example, they found that arsenic 
trisulfide hydrosol precipitates Odén’s sulfur hydrosol, both of 


11 Kolloid-Z., 6 (1910), 78. 
12 Kolloid-Z., 28 (1920), 258; 29 (1921), 16. 


328 COLLOIDAL BEHAVIOR 


which migrate to the anode in an electrical field. Obviously, 
the electrical charge neutralization theory fails in this instance. 
Since Odén’s sulfur sol contains pentathionic acid as stabilizing 














TaBLe IV 
Positive sol Negative sol 

Agl AgNO; KI AglI 
1 0.210 0.052 0.055 0.220 
2 0.220 0.055 0.050 0.200 
3 | 0.225 0.009 0.010 0.245 
4 | 0.245 0.010 0.017 0.180 
5 0.270 0.009 0.010 0.040 
6 0.240 0.010 0.007 0.033 
7 0.038 0.007 0.010 0.240 
8 0.040 0.010 0.008 0.196 
9 0.115 0.010 0.010 0.240 
10 0.115 0.010 0.007 0.170 
11 0.225 0.009 0.010 021415 
12 | 0.240 0.010 0.008 0.088 














agent (or as one of its stabilizing agents) and arsenious sulfide 
sol is stabilized by hydrogen sulfide, it was deduced that the 
mutual precipitation of these sols is a result of the following 
chemical reaction between their stabilizing agents: 


5 H.S 4 H.S;0¢ — 10S + 6H:O 


It was shown further by these investigators that the following 
combinations of negatively charged hydrosols result in coagula- 
tion: Odén’s sulfur and Carey Lea’s silver; von Weimarn’s sulfur 
(made by pouring an alcoholic solution of sulfur into water) and 
Carey Lea’s silver; Odén’s sulfur and Kruyt’s selenium. When 
Carey Lea’s silver sol is mixed with arsenious sulfide hydrosol 
the mixture turns brown, and, if left in the dark, no further change 
takes place. Upon exposure to light, however, it undergoes a 
rapid change to olive green — emerald green — orange yellow 
(still clear), and then becomes turbid. The mechanism of this 
phenomenon is not understood. 


MUTUAL REACTIONS OF COLLOIDS > oad 


Recently a careful quantitative study of mutual precipitation 
of ferric oxide hydrosol by silica hydrosol has been reported by 
Thomas and Johnson.!* This paper offers strong evidence to the 
effect that mutual precipitation of certain hydrosols is the result 
of chemical reaction between the ions of the respective stabilizing 
electrolytes present, to which, in accordance with the ‘‘ Complex 
Theory” of colloids, certain colloids owe their stability or solu- 
tion-attractive forces. 

To prove this point they were obliged to select sols which 
could be analyzed for the content of stabilizing agent. Instead 
of reporting merely the amounts of the insoluble part of the dis- 
persed phases, as was customary formerly (and really of little 
or no significance), the amounts of stabilizing electrolytes are given 
by them. 

The preparation and composition of the sols used are given 
in Tables V and VI. 


TasBLE V.—ComposiITION oF FERRIC OxIDE SOLS 











No. Mols FeCl; Mols Fe.0; Fe.,0;3/FeCls 
1 0.00895 0.0482 5.35/1 
2 0.01075 0.0509 4.8/1 
3 0.00889 0.0483 Dott 
4 0.00338 0.0294 8.65/1 
6 0.00178 0.0191 10.6/1 

15 0.00518 0.0507 9.8/1 

17 0.00267 0.0346 130/71 

20 0.00145 0.0267 183571 

21 0.00230 0.0367 15.9/1 

23 0.00238 0.0339 14.3/1 


Silicic acid sols were prepared by dissolving water glass (d., 
1.4) in 10 times its weight of water and partially neutralizing to 
various degrees with hydrochloric acid. The sodium chloride 
formed was not removed because (1) the freshly prepared sols 
diffused quite readily through unglazed porcelain dialyzers and 


13. J. Am. Chem. Soc., 45 (1923), 2532; Colloid Symposium Monograph, 
Univ. of Wisconsin, 1923, p. 187. 


330 COLLOIDAL BEHAVIOR 


through ordinary collodion sacks; these sols after standing showed 
aggregation of particles (became opalescent) and then would not 
diffuse through the septa mentioned; but removal of sodium 
chloride by dialysis would also remove sodium hydroxide formed 
by hydrolysis of sodium silicate, which was not desired; (2) 
the presence of the sodium chloride did not complicate the 
results, as shown later. 


TaBLE VI.—ComMPOSITION OF SiLicic Actp SOLS 
In mols per liter 





ING ahs Risers eer eee 1 2 3 4 5 

SiO es. Samia 1s ate oe 0.221 0.208 0.305 | 0.437 | 0.186 
NaOH sat. ipod ene 0.0327 | 0.0356 | 0.0294 | 0.185 | 0.0115 
Bis NAC Toe. i Garyit 5.8/1 10.4/1 3. 5/1 eG At 
NaCl? Stehs rae ce ten ae 0.091 0.081 0.142 0.111 | 0.093 











Various amounts of one sol were run from a burette into hard 
glass bacteriological tubes of uniform diameter and clearness and 
about 20 cc. in volume. Equal volumes of the other sol were 
run from a pipette or burette into the tube as quickly and as 
quietly as possible, to avoid mixing. The tubes were inverted 
two or three times with as little agitation as possible, to insure 
complete mixing of the sols. This was sufficient if the tubes 
were not more than three-fourths full and were revolved as they 
were inverted. The point of maximum precipitation could be 
clearly seen by successive stages; first, the greatest cloudiness in 
the series, followed by the first separation of particles, and usually 
the first sediment. The last was frequently over a wider range 
than the first two stages as the precipitating zone widened rapidly. 
All observations were made immediately after mixing by holding 
the series of tubes against a clean or open window. Artificial 
light was found to be quite unsatisfactory, as was the light late 
in the afternoon or on a dark day. 

Some series precipitated more easily than others, probably 
because of greater agitation. Vigorous shaking usually precipi- 
tated the whole series immediately. A tube of greater diameter 


Bigresines 


MUTUAL REACTIONS OF COLLOIDS 331 


than the others or of cloudy or marked glass appeared more 
cloudy and, consequently, was misleading. A few series were 
centrifuged, but the time required in placing them in the centri- 
fuge and the force to throw out the fine particles resulted in 
complete precipitation over a wide range. 

To avoid errors due to an unclean tube or some other factor, 
three or four determinations were made in each instance. 

The effect of dilution of the sols is given in Table VII. 


TasLE VII.—Errect or DinutTion urPpon MutTuaAtL PRECIPITATION 









































Millimols 
Ce. Ce. Precipitation 
Fe.03 sol No. 6 | SiOz No. 1 results 
HCl|* NaOH* 
Le Senta 1:10 
5.3 1to 10 | All coagulated to solid 
2. 1:10 | 1:50 
5.3 3.0 Partial 
5.3 3.5 Almost complete 
5 3 4] Coraplais 0.00284 | 0.00268 
5.3 4.5 Almost complete 
3 1:50 12250 
5.3 4.5 Partial 
5.3 4.8 Complete 
5 3 ct 4.9 en nies 0.00057 | 0.00064 
5.3 5.4 Partial 
4, 1: 100 12250 
10 4.7 Much slower 
10 5.0 First to precipitate 
10 5.3 Much slower Re ee 
10 D6 Much slower 














* The figures under the headings ‘‘HCl” and ‘‘NaOH” in the table express the amounts 
of ferric chloride and sodium silicate in terms of their hydrolysis products, HCl and NaOH, 
present in the respective sols in the mixture which resulted in first or complete precipitation. 

This table shows that the dilution of mutually precipitating 
sols narrows and sharpens the zone of maximum precipitation, 
and that the variation in ratio between the sols varies no more than 
in successive experiments with the same dilution. If carried 


332 COLLOIDAL BEHAVIOR 


beyond certain dilutions, coagulation is almost imperceptible 
or is so slow that it is very difficult to determine. The dilutions 
used for the work were the greatest giving sharp precipitations. 
Very unstable sols show considerably different ratios with dilu- 
tion. This will be discussed later. 

To avoid possible differences due to order of mixing, the order 
was always reversed at least once. 

Table VIII gives the data found in determining the points 
of maximum precipitation in cases typical of 37 experiments 
made, and Table IX shows results obtained with one silica sol 
and several ferric oxide sols, which are typical of two other 
similar series. 


TABLE VIIIA.—Murtvau PRECIPITATION OF SiLticic Actip Hyprosou No. 1 
witH Ferric OxipE Hyprosou No. 20 ; 
Dilutions: Fe,O3; sol = 1:25 SiO, sol = 1:200 
5.3 ec. of ferric oxide sol added to silicic acid sol 


























First test | Second test 
Ce. i Oe? 
Si. Bol Results Oy sol Results 
4.0 Partial precipitation 4.5 Partial later precipitation 
5.0 First and heaviest precipi- | 
tate 0.0 7 cat 
6.0 Partial precipitation 5s ae 
7.0 Only slightly cloudy 6.0 Partial later precipitation 








It was evident that 5.2 cc. of silicic acid sol with 5.3 cc. of 
ferric oxide sol give the maximum precipitation. 


TasLE VIII 
Silicic acid sol added to 5.3 cc. of ferric oxide 














Cc, Ce. 

SiO! Results SiO. eal Results 
4.8 Fourth to precipitate 5.4 Second to precipitate 
5.0 | Second to precipitate 5.6 Third to precipitate 
5.2 First to precipitate 

















Cy apes, SI oath tie WL ae 


Let Bes Sn a 


MUTUAL REACTIONS OF COLLOIDS 338 


Maximum precipitation is obtained with 5.2 cc. of silicic acid 
sol and 5.3 cc. of ferric oxide sol. 


TasLE [X.—TuHeEe Ratio at Maximum PRECIPITATION OF Siuicic Acrp 
Sot No. 1 to DirrerEnt Ferric OxipE SoLs 











Fe.03 sol Milli-equivalents | SiO, sol 
No. — = aa SRR : 
Dilution Ce. Chloride | Sodium Ce. Dilution 
1 TELA oi 10.0 | 0.00199 | 0.00151 4.6 1:100 
2 1:100 ps: 0.00171 | 0.00124 3.8 1:100 
3 1:100 10.0 0.00267 | 0.00180 5.5 1:100 
4 1:100 10.0 0.00102 | 0.00112 oOo 1:100 
6 1:100 10.0 0.00054 | 0.00065 5.0 1:250 
15 1:50 ao 0.00165 | 0.00155 9.5 1:200 
ay 150 5.3 0.00085 | 0.00085 tae 1:200 
20 1325 Be3 0.00085 | 0.00085 532 1:200 
34) Leh 5.3 0.00147 | 0.00164 10.0 1:200 
23 1:25 5.3 0.00151 | 0.00155 9.5 1:200 




















Silicic acid sol No. 4 was precipitated at different dilutions 
against ferric oxide sols Nos. 4, 6, and 20. No clear zones could 
be determined, but in every case the milli-equivalents of sodium 
exceeded the milli-equivalents of chloride. 

The precipitating volumes checked within 10 per cent in all 
cases except that of the very impure silicic acid sol No. 4. 

To test the effect of the sodium chloride present upon the point 
of maximum precipitation, 0.03 g. of sodium chloride (an amount 
equal to 60 per cent of the amount of sodium chloride already 
present in the sol) was added to silicic acid sol No. 2 (diluted 
1:200), and the sol precipitated with several ferric oxide sols. 

The results showed that the sodium chloride is without influ- 
ence on the mutual precipitation of silicic acid sols and ferric 
oxide sols, and, consequently, no error was introduced by its 
presence in the silicic acid sols used. 


334 COLLOIDAL BEHAVIOR 


TapLe X.—TueE Ratio or Ferric CHLORIDE EXPRESSED IN EQUIVALENTS 
or Hyprocuioric AcID To SopiuM SILICATE 


Expressed in equivalents of sodium hydroxide in the 37 precipitations of 
ferric oxide sol and silicic acid cited in the foregoing tables. ‘The values 
are milli-equivalents of sodium hydroxide per 0.001 milli-equivalents of 
hydrochloric acid. 

The sols are arranged in the order of their purity, the ratio of sodium 
silicate in terms of sodium hydroxide to silicon dioxide decreasing from left 
to right, and the ratio of ferric chloride to ferric oxide decreasing from top 
to bottom. 


ee 


Silicic acid sols 



































Number | 4 2 sf | 3 | 5 
Mols NaOH/SiO. | 1/3.5 | 1/5.8 | 1/6.7 | 1/10.4 | 1/16.1 
Ferric oxide sols 

No Mols 
FeCl;/Fe203 
Py lf 453 eee 0.00061 | 0.00073 | 0.00080 
1 1/3185 VOR ee 0.00080 | 0.00055 | 0.00077 
3 Ly bso 7 on a eeeerats 0.00073 | 0.00069 | 0.00077 
4 1/ 8.65 >100% | 0.00096 | 0.00091 | 0.00090 
15 1/°9:8)% Ache 0.00090 | 0.00100 | 0.00080 | 0.00029 
6 1/10.6 >100% | 0.00106 | 0.00094 | 0.00082 
17. 17.13.04) eee 0.00101 | 0.00090 | 0.00080 | 0.00043 
23. 1/1403 yeaa 0.00101 | 0.00091 | 0.00090 | 0.00048 
pat 1/15.9 >100% | 0.00098 | 0.00110 | 0.00080 | 0.00044 
20 1/1852. fia eee 0.00093 | 0.00100 | 0.00080 
Ay. (excl. of 2,-1,-3) >100% | 0.00098 | 0.00095 | 0.00083 | 0.00041 











Table X shows that over a wide range of ratios between peptiz- 
ing agent and dispersed phases of sols there is a constant ratio 
between the peptizing agents of mutually precipitating sols and a 
greatly varying ratio between the insoluble part of the dispersed 
phases. This leads to the conclusion that the precipitation is due 
to removal of the peptizing agents by a chemical action between 
them. As no definite formulas can be given for the silicates, it is 
impossible to express the reaction between sodium silicate and 
ferric chloridein anequation. However, the peptizing ferric chlor- 
ide and sodium silicate can be given in terms of their hydrolysis 





MUTUAL REACTIONS OF COLLOIDS 335 


products which are in equilibrium with the dispersed phase com- 
plexes, and, consequently, in the mutual precipitation of ferric 
oxide and silicic acid sols the reaction may be expressed as HC] 
+ NaOH — NaCl + H.0. 

Another indication of such a reaction is given by the difference 
of hydrogen ion concentration in the supernatant liquid of pre- 
cipitation of varying degrees. Various amounts of silicic acid 
sol No. 3 (diluted 1:25) were added to 25 cc. of ferric oxide sol 
No. 4 (diluted 1:10), the precipitates allowed to settle, and the 
hydrogen ion concentrations of the supernatant liquids determined 
with the results given in Table XI. The ferric oxide sol showed 
a Sérensen value (pH) of 5, while the silicic oxide sol was slightly 
alkaline to phenolphthalein. 


TaBLE XI.—CHANGE IN HypROGEN ION CoNCENTRATION WITH VARYING 
PRECIPITATION OF FERRIC OXIDE AND Srrictc Acip Sots 


| 


Silicic acid 





een | Remarks pH 

10 Very slight precipitation | Deu 

Excess of Fe203; sol 
16 Maximum precipitation 6.8 
18 Second of series to pre- has 
cipitate 
20 Slow precipitation | Excess of SiO: sol > 8.3 (alkaline 
to phenolphthalein) 





Thus it is evident that maximum precipitation occurs at 
neutrality as the chemical equivalence of the peptizing electrolyte 
demands and that the acidity increases with an excess of ferric 
oxide sol, and the alkalinity increases with an excess of silicic acid 
sol. 

’ The ferric oxide sols, excluding Nos. 1, 2, and 3 (which contain 
the greatest amount of the peptizing agent in proportion to the 
dispersed phase), give a 1:1 ratio with silicic acid sols Nos. 1 and 
2. With silicic acid sol No. 3, the same ferric oxide sols give 
a constant ratio with the ferric chloride of the same ferric oxide 
sols. As the sol becomes purer in respect to peptizing agent, 


336 COLLOIDAL BEHAVIOR 


it becomes more unstable and the precipitating ratios become 
more inconstant. This agrees with the general experience with 
“pure”? ferric oxide sols. It is known that when a certain 
degree of “‘purity’’ is exceeded in a sol, the sol precipitates. The 
sol containing only a little more than the necessary minimum 
of peptizing agent is in a metastable condition, and the least 
disturbance will precipitate it. This may easily account for the 
ready precipitation of the pure ferric oxide sols. 

The effect of diluting unstable sols in mutual precipitation is 
shown in Table XII. 


TaspLe XII.—PRECIPITATION OF FERRIC OxiwE Sou No. 17 witH SILIcic 
Acip Sot No. 5 oF Various DILUTIONS 


ee ee eee ee 





Fe,O3 sol, 1:50 Milli-equivalents of HCl, 0.00160 
SiQg. 801) civaee pee eee 1:50 1:100 1:200 ° 
NaOH milli-equivalents........ 0.00057 0.00069 0.00080 
NaQH eq. of HCl eq. 952 as 30. 43. 50. 











a 


Thus it is seen that the reaction between very “‘pure”’ sols 
tends to approach chemical equivalence upon dilution, which 
is to be expected, since the individual particles becoming widely 
separated upon dilution have less chance of aggregating and thus 
settling out. In the meantime there is more chance for a com- 
plete reaction between the peptizing agents. 

Both ferric oxide and silicic acid sols show erratic results in 
precipitation, if they contain large amounts of peptizing agent. 
This is undoubtedly due to the fact that some of the unaffected 
peptizing agent is adsorbed in the coagulum and carried down 
with it. 

Thomas and Johnson tried also to study the mutual precipi- 
tation of ferric oxide and arsenious sulfide sols as a function of 
the reaction between the stabilizing ferric chloride of the iron 
oxide sol and of the stabilizing hydrogen sulfide of the arsenious 
sulfide sol. The errors existing in the present quantitative 
methods for the determinations of arsenic and of sulfur are large 
enough to make an analysis of the vAs2S;.yH25 complex impos- 


: 





MUTUAL REACTIONS OF COLLOIDS 337 


sible, due to the small amounts involved. They reasoned that 
if, in the mutual precipitation of ferric oxide sol and arsenious 
sulfide sol, a chemical reaction takes place between the ferric 
chloride and hydrogen sulfide of peptization, one of the following 
reactions may take place: (1) H.S + 2FeCl; — 2FeCl., + S + 
2HCl; (2) 3HS + 2FeCl; — 2FeS + S + 6HCI. 

In most precipitations there is no evidence of the latter reac- 
tion, as the precipitate is yellow. When, however, the sol 
contains a large amount of hydrogen sulfide, a blackening 
develops which can be explained by the formation of ferrous 
sulfide. 

To test the supposition that the mutual precipitation between 
ferric oxide sol and arsenious sulfide sol is due to the oxidation 
of the sulfide ion of the peptizing hydrogen sulfide by the ferric 
ion of the peptizing ferric chloride, the following experiment 
was performed. 

Five hundred cubic centimeters of an arsenic trisulfide sol 
were precipitated with ferric oxide sol No. 4, the precipitate dried 
and extracted with carbon disulfide. Sulfur was recovered. 
Since arsenious sulfide sols are most likely to contain sulfur after 
exposure to air, a ‘“‘blank’’ was run wherein an equal quantity 
of sol was precipitated by aluminum sulfate, the precipitate 
dried, extracted, and the sulfur found subtracted from that 
obtained in the main experiment. | 

The presence of sulfur in the gel (in excess of the small amount 
originally present in the sulfide sol) can be accounted for only 
through the following chemical reaction 


S= + 2Fet++ — 8° + 2Fet+ 


In view of the evidence submitted by Freundlich and Nathan- 
sohn and by Thomas and Johnson, it would appear that the older 
electrical charge theory of colloid interaction must give way to 
the chemical reaction hypothesis. 

The degree of dispersion of the interacting colloids has been 
claimed to be significant by Galecki and Katorski.14 They 
state that the precipitating power of one colloid upon another 
increases with increasing degree of dispersion, and believe they 
have demonstrated this fact by the reaction of a ferric oxide 


14 Kolloid-Z., 18 (1913), 43. 


338 COLLOIDAL BEHAVIOR 


hydrosol with (1) a gold hydrosol made by the formaldehyde 
reduction method (Aug), and (2) a gold hydrosol prepared by 
reduction with phosphorus (Aup). Ultramicroscopic observa- 
tions showed the Aup sol to be of higher degree of dispersion than 
the Aup. The ferric oxide sol which they used contained 10.28 
mg. Fe.O; per cubic centimeter. 


1 mg. Aur precipitated 4.98 mg. Fe2Os. 
1 mg. Aup precipitated 18.36 mg. Fe20s. 
When the ferric oxide sol was diluted three times, 
1 mg. Aug precipitated 4.65 mg. Fe.Os. 
1 mg. Aup precipitated 20.65 mg. Fe20s. 


At first glance their claim appears plausible, but when one 
considers that these gold sols are prepared from gold chloride, 
sodium carbonate, and varying amounts of formaldehyde or of 
phosphorus, resulting not only in reduction of the auric ion but in 
oxidation of the reducing agents, forming different amounts and 
kinds of products in each instance, it is evident that such a simple 
mechanical explanation is of doubtful value. 

Hydrophilic colloids, such as the proteins, mutually precipi- 
tate under certain conditions. Kutscher!® and Bang'® showed 
that protamine precipitates other proteins. Malengreau?’ found 
that histone mutually precipitates with hemoglobin, serum albu- 
min, and globulin. 

The significance of the hydrogen ion concentration of the solu- 
tion in the mutual precipitation of proteins was demonstrated by 
Michaelis and Davidsohn.!8 They stated that when two ampho- 
teric colloids, such as proteins, are brought together in solution, 
a compound may be formed and precipitated, the condition for 
most complete precipitation being a hydrogen ion concentration 
between those of the isoelectric points of the reacting ampholytes. 
Thus, when one protein is present as a cation and the other as an 
anion, the formation of a compound is to be expected, whereas 
when both proteins are cations (the pH is acid to both of their 
isoelectric points), or where both are anions (the pH is on the 


15 Z, physiol. Chem., 23 (1897), 117. 
16 Tbid., 27 (1899), 483. 

17 Le Cellule, 21 (1903), 121. 

18 Biochem. Z., 39 (1912), 496. 





MUTUAL REACTIONS OF COLLOIDS 339 


alkaline side of both isoelectric points), a combination between 
them is not to be expected. The combination between pairs of 
proteins has resulted in insoluble complexes in practically 
all cases tried, but, since a great deal of work has yet to be done 
on this subject, it is not safe, at present, to state that proteins 
always precipitate one another if mixed together at a pH between 
their isoelectric points. Michaelis and Davidsohn found also that 
the optimum pH for the mutual precipitation of proteins varies 
with the relative amounts of the proteins reacting; when a large 
excess of one component is present, the pH optimum for precipita- 
tion will shift toward the isoelectric point of this component. 

The precipitation optimum for a mixture of aqueous dispersions 
of nucleic acid and serum albumin was found to be at pH 4.05 
to 4.22, which is between the isoelectric points of the compo- 
nents, while a mixture of nucleic acid and heat-denatured serum 
albumin precipitated best at pH 3.8. When the ratio of 
nucleic acid to the albumin was increased, the optimum reaction 
for precipitation shifted toward the acid side, 7.e., toward the 
isoelectric point of nucleic acid. 

Casein and nucleic acid precipitated each other in a pH range 
of 4.05 to 2.52, depending upon whether casein or nucleic acid 
was present in excess. Mixtures of casein with both genuine and 
denatured serum albumin were found to result in precipitation. 
Variation in the mass relationships made no difference, due to 
the fact that the isoelectric points of these proteins are so close 
together. 

Beth af Ugglas!® reported the mutual reactions between 
clupein, thymushistone, casein, and horse blood hemoglobin. 
The protamin precipitates hemoglobin, the coagulum being 
-peptized by excess of hemoglobin and by acid. The coagula 
obtained always had the composition of 95 per cent hemoglobin 
and 5 per cent protamin. Histone acts like the protamin on 
hemoglobin, the coagulum consisting of one part histone to two 
parts of hemoglobin. In both cases the dissolution of the coagu- 
lum by acid was reversed upon addition of ammonia. 

The precipitation of casein by protamin as shown by Hunter” 
was also studied by af Ugglas, who found that a ‘‘neutral”’ 


12 Biochem. Z., 61 (1914), 469. 
20 Z, physiol. Chem., 53 (1907), 526. 


340 COLLOIDAL BEHAVIOR 


protamin precipitated a “neutral” caseinate. The coagulum is 
insoluble in cold water, but soluble in warm water and in satu- 
rated sodium chloride solution. It is also peptized by excess of 
casein. The composition of the coagulum was about 94 per cent 
casein.and 6 per cent protamin. Histone was found to act upon 
casein just like the protamin; the precipitate dissolving in dilute 
alkali and in excess of casein. When dissolved in alkali and repre- 
cipitated by acid, the coagulum showed no alteration in composi- 
tion from the ratio of 71 per cent casein and 29 per cent histone. 

Hemoglobin and casein were found not to react in slightly 
alkaline solution, but precipitated each other in “neutral” 
solution, the coagulum dissolving in dilute alkaline solution 
and reprecipitating upon acidification. Af Ugglas found the 
coagulum to be composed of two parts of hemoglobin to one of 
casein. 

In the experiments reported by af Ugglas the significance of 
the hydrogen ion concentration of the solutions in the precipita- 
tions is evident, although unfortunately not measured by the 
investigator. 

Based upon 16,700 as the molecular weight of hemoglobin, and 
the value of 8,888 for casein, as given by van Slyke and Bos- 
worth,2! the casein hemoglobin precipitate consisted of one 
molecule of casein to one of hemoglobin. Assuming 6,122 as 
the molecular weight of histone,?? a ratio of two molecules of 
casein to one of histone would call for a composition of 26 per cent 
histone and 74 per cent casein. Analysis of the coagulum showed 
29 and 71 per cent respectively. 

Michaelis and Davidsohn?? investigated the influence of 
pH in specific precipitations. Using as precipitin the serum of a 
rabbit that had been previously sensitized with sheep serum, 
flocculation of this precipitin and sheep serum was obtained 
equally as well at pH 9 as at pH 5, thus showing no dependence 
upon the hydrogen ion concentration. ‘This sort of reaction is 
then different from protein mutual precipitation. 

De Kruif and Northrup™ have recently shown the same to be 
true for the agglutination of Bacillus typhosus by immune serum. 


21 J, Biol. Chem., 14 (1913), 227. 

22 Bana, Beitr. chem. Physiol. Path., 4 (1903), 348. 
23 Biochem. Z., 47 (1912), 59. 

24 J. Gen. Physiol., 5 (1922), 127. 


MUTUAL REACTIONS OF COLLOIDS 34] 


They found that the amount of immune body combined with 
the organisms is constant from pH 9 to pH 3.7, and that the 
combination is not caused by a difference in sign of the charge 
carried by the immune body and the organism. 

The flocculation of bacteria by proteins, however, has been 
found to be similar to protein mutual precipitations. Eggerth 
and Bellows* found that a suspension of Bactertwm coli is agglu- 
tinated by gelatin, crystallized egg albumin, proteoses, edestin, 
and oxyhemoglobin at hydrogen ion concentrations between the 
isoelectric point of the protein and the acid flocculation zone of 
the bacterial suspensions, the latter having been found to lie 
between pH 1.6 and 3.0. The following table is typical of a 
series reported by these investigators. 


TaBLE XIII.—FtLoccunation or B. Coli SUSPENSION WITH Eaa 
ALBUMIN 


1.0 ce. buffer mixture + 0.5 ec. albumin solution + 0.5 ce. B. coli suspension 


Concentration 


. ff a 
of albumin Lactate buffers 








0 = a om 
1: 400,000 = — = 
1: 40,000 — 
1: 4,000 — 
1:400 co 


| 
| 
| 





| Pat | 
[ we 
444 
ae 
| 


| 


md | 
PD ed | 









































pH = 4.7 | 4.4 | 4.1 3.8 3.5 3.3 rein Fs aay 


Temperature = 40°C. X = agglutination within 1 hour. + = agglutination within 
4 hours. 


A second strain of the organism was agglutinated at pH 4.7 
by an albumin concentration of 1:150. It is seen that as the 
ratio of albumin to bacteria increases, the optimum flocculation 
point shifts toward the isoelectric point of the albumin. 

The proteins of blood serum are precipitated by lecithin 
suspensions at hydrogen ion concentrations between the iso- 
electric points of the reacting substances. The isoelectric points 


% J. Gen. Physiol., 4 (1922), 669, 


342 COLLOIDAL BEHAVIOR 


(or flocculation optima) of lecithins have been found to vary 
from pH 2 to 4, depending upon the source.2® As the pro- 
portion of blood serum to lecithin is increased, the optimum pre- 
cipitation tends to shift toward the reaction of the isoelectric 
point of the blood serum proteins. 

Jarisch”’ also finds precipitation between lecithin and dialyzed 
blood serum. Soaps and colloidal suspensions of fatty acids also 
cause flocculation of a protein in dialyzed blood serum, but 
not in the presence of 0.012 N sodium chloride. 

An interesting example of the mutual precipitation of hydro- 
philic or “protective” colloids is that of gelatin with gum arabic. 
Gum arabic appears to consist mainly of a complex carbohydrate 
acid combined with more or less calcium as a calcium salt. In 
view.of the recent work of Jacques Loeb, one would expect a 
precipitation of gelatin by gum arabic in solutions on the acid 
side of pH4.7 (the isoelectric point of gelatin), if gelatin 
“arabate”’ is an insoluble complex. | 

Thomas Graham (1861) showed that gelatin is precipitated by 
“gummic acid,” the coagulum settling out to form a jelly-like 
mass. ‘This reaction has been rediscovered by Tiebackx,”8 
whose attention to it was aroused by the fact that oil-in-water 
emulsions “broke”? upon mixing if one was emulsified with 
gum arabic and if gelatin was the emulsifying agent in the other. 
He found that gelatin and gum arabic mutually precipitate in a 
solution sufficiently acid to insure the presence of gelatin cations, 
the coagulum setting to a jelly when warmed. In the presence 
of an excess of gelatin this precipitation does not occur. This 
is an example of the ‘‘protective”’ effect of an excess of one com- 
ponent as seen in the mutual precipitation of inorganic colloids 
discussed earlier in this chapter, and will be referred to again 
later. Tiebackx noted that gum tragacanth precipitates gelatin. 
The flocculation of gelatin by gum arabic has been reported also 
by Luppo-Cramer. 7 

The precipitation of proteins by tannin may properly be 
included here, since it is generally agreed that tannin is more 


26 FEINSCHMIDT: Biochem. Z., 38 (1912), 244. 

7 Klin. Wochschr., 1 (1922), 71 (through Chem. Abs., 16 (1922), 2871). 
28 Kolloid-Z., 8 (1911), 198, 238; 31 (1922), 102. 

2° Phot. Korr., 61 (1918), 111. 


MUTUAL REACTIONS OF COLLOIDS 343 


colloidal than “‘erystalloidal” in nature. Tannin is a well-known 
protein precipitant, one being used as a test for the other. The 
precipitation of gelatin by tannin has been studied rather exten- 
sively, and, of course, the reaction between tannins and hide 
protein in leather manufacture, but, since the latter protein is 
insoluble, a discussion of its combination with tannin is outside 
the realm of this chapter. 

The discovery of the gelatin-tannin coagulation is attributed 
to Seguin,®° although Seymour-Jones*! claims that, in 1762, 
Lewis found that galls contained an astringent substance capable 
of precipitating gelatin from solution. The reaction was studied 
from a quantitative point of view by Humphrey Davy. *? 

An investigation has been recently published by Thomas and 
Frieden.** They found that the optimum precipitation of gelatin 
and tannic acid, in the absence of salts, takes place at pH* 
44to4.6. The effect of pH is shown in Table XIV. It is seen 
that maximum precipitation is obtained on the acid side of the 














TaBLE XIV 

Appearance of | Volume of 

pH supernatant precipitate, 
solution cubic centimeters 

3.9 Clear, yellowish 0.90 
4.1 Clear, yellowish 1.10 
4.3 Clear, yellowish leao 
4.5 Clear, yellowish 1.20 
4.7 Cloudy 0.90 
4.9 Milky 0.80 
cet Opalescent 0 
5.3 Slightly opalescent 0 





isolectric point of the protein, an insoluble compound being 
formed through the interaction of gelatin cations and tannin 
anions. The optimum at a hydrogen ion concentration just 


30 Ann. chim., 20 (1796), 15. 

31 J, Soc. Leather Trades Chem., 4 (1920), 119. 
32 Phil. Trans., 93 (1803), 233. 

33 Ind. Eng. Chem., 15 (1923), 839. 


344 COLLOIDAL BEHAVIOR 


slightly above that of the isoelectric point of the gelatin followed 
by a drop at higher acidities is explainable due to the fact that 
tannic acid is an exceedingly weak acid. A precipitation slightly 
on the alkaline side of the isoelectric point of protein is not unex- 
pected, since the ionization of gelatin as a base has not vanished 
at pH 4.9. It must be borne in mind that a protein is ionized 
both as a base and an acid at its isoelectric point. The view 
that it is completely un-ionized at this point is wrong. It is 
ionized but its basic and acidic dissociations are equal in extent, 
and as the alkalinity (or acidity) is increased, its basic (or acidic) 
degree of dissociation decreases and its power to combine with 
acids (or bases) likewise decreases. 

The influence of the relative proportions of gelatin and tannin 
are shown in Table XV, where the reaction was maintained at 
pH 4. 














TaBLE XV 
; Volume of Test of supernatant 
Ratio of precipitate, Appearance of liquid for 
tannin to eihin supernatant 
Seca centimeters Hatad Gelatin Tannin 
20 0.4 Clear, yellow _~ = 
10 0.9 Clear, yellow o -- 
8 0.9 Clear, yellow — ++ 
6 1.4 Clear, slight yellow _ + 
4 1S Clear, slight yellow — + 
2 2.5 Clear, colorless = Almost — 
1 1.8 Milky + 








The best precipitation is obtained at a ratio of two parts tannin 
to one of gelatin. An excess of either results in “ peptization,” 
just as in the case of inorganic colloids. The importance of the 
correct ratio of gelatin to tannin is shown by the misstatement of 
Michaelis and Davidsohn*‘ to the effect that the optimum pH 
varies from 3.8 to 5.7. This was due to the fact that, in a number 
of the few experiments tried by them, there was an excess of 
gelatin. 


34 Biochem. Z., 54 (1914), 323. 





MUTUAL REACTIONS OF COLLOIDS 345 


Thomas and Frieden found that certain vegetable tannin 
extracts acted differently from pure tannic acid in respect to 
optimum hydrogen ion concentration. This is shown in Table 
mV 1. 


TABLE XVI.—OptTimumM PH RANGE FOR TANNIN-GELATIN PRECIPITATION 


Extract | pH 
a | 42010325" 
ES ie 3.5 to 4.0 
Be re en ee ciel boxe p ce ne ens 4.0 to 4.5 
SRNR NG ee ee ne eww a 4.0 to 4.5 
rented iw ea | 4.5 to 4.0 
phe a OE en eee 8:5 to 4.0 





* The optimum reaction is given last in each case. 


An interesting comparison of the delicacy of the reaction 
between commercial tannins and gelatin at optimum pH and in 
distilled water with no pH control is exhibited in Table XVII. 


TaBLE XVII 











pe intdictillediwater 
xtract at optimum pH «1 bj 
*1 part in ae tes 
1 VOWS 2 hs AG eae cr 150 , 000 20,000 
My oa shade aE. Rass 150,000 7,500 
MMe Me eek, ces dhe oata s 130,000 6 , 500 
EN tO) oh. a 130,000 17 , 000 
eh Mar re 8 ee le 200 , 000 20,000 
SERRE hE aa 110,000 30, 000 





* Parts tannin in parts water. 


The significance of pH control is well illustrated in the table 
above. When the reaction is used as a test either for a protein 
or for tannin, the addition of sodium chloride will broaden the 
pH range of precipitation and thus counterbalance to an extent 
a lack of pH regulation, although it will not increase the sensitivity 
of the precipitation at the optimum pH. 


346 COLLOIDAL BEHAVIOR 


PROTECTIVE ACTION 


When a solution of a hydrophilic colloid is added to a less 
stable colloidal dispersion, or suspension, generally there is no 
change in appearance of the system and the less stable dispersion 
is found to have become more stable, 7.e., it is no longer so sensi- 
tive toward coagulation by either the addition. of electrolytes 
or by evaporation to dryness. The less stable dispersion is said 
to have been “‘protected”’ by the hydrophilic colloid; hence the 
term ‘protective colloid,” which is commonly applied to the 
hydrophilic colloids, such as gelatin, gum arabic, albumin, ete. 

The discovery of protective action may justly be attributed to 
Michael Faraday, who noted that the addition of gelatin to his 
colloidal gold dispersions rendered them so stable that it was 
possible to evaporate them to dryness without change in color.*® 

Since there appears to be a general tendency to regard “ pro- 
tective”’ colloids as a class that always confers increased stability 
upon lyophobic colloids, it would be well to stop for a moment in 
order to show that “protective” colloids do not differ so radically 
from others in their conduct in mutual reactions. It is more a 
difference in degree than in kind. For example, hydrophilic 
colloids may precipitate other dispersions. We have just 
reviewed a number of instances where certain protective colloids 
precipitate each other. 

The precipitation of alumina hydrosol by gelatin was observed 
by Thomas Graham. This thoroughgoing scientist also describes 
the mutual precipitation of colloidal silica by gelatin. Itis 
interesting to note that he attempted to follow this reaction 
quantitatively as shown in the following quotation from his 
paper: 

Silicate of gelatin falls as a flaky, white, and opaque substance, 
when the solution of silicic acid is added gradually to a solution of 
gelatin in excess. The precipitate is insoluble in water and is not 
decomposed by washing. Silicate of gelatin prepared in the manner 
described contains 100 silicic acid to about 92 gelatin. In the humid 
state the gelatin of this compound does not putrefy. When a solution 
of gelatin was poured into silicic acid in excess, the cosilicate of gelatin 
formed gave, upon analysis, 100 silicic acid with 56 gelatin. 


% Phil. Trans., 147 (1857), 184. 
3% J, Chem, Soc., 15 (1862), 246, 





MUTUAL REACTIONS OF COLLOIDS 347 


This appears to have been overlooked, since in recent colloid 
literature one notes reports of the discovery that sometimes 
protective colloids do not protect. One such report is that of 
Brossa and Freundlich.*” These authors find that the addition 
of a small amount of well-dialyzed albumin solution to ferric 
oxide hydrosol renders the latter more sensitive toward the 
precipitating influence of electrolytes rather than more stable. 
The explanation for this is simple, and will be returned to later. 
It is evident that the findings of Graham were overlooked as 
well as the more recent papers of Friedmann* and of Pauli and 
Flecker.*® 

Friedmann noted that albumin, when used in the proper propor- 
tion, would precipitate hydrosols of Ag, AseS3, SbeS2, Si02, MoOz, 
Fe.O3, and Cr2O3. Pauli and Flecker carried out a large number 
of experiments on the coagulation of a series of inorganic colloids 
by serum proteins and gelatin. 

Protective colloids may protect less stable dispersions or may 
render then still less stable, even resulting in mutual precipita- 
tion, depending upon the signs of the charges of the protector and 
hydrophobe, and upon the relative proportions of the two sols 
brought together. The significance of the signs of the charges 
carried by the two colloids interacting was shown by Billitzer.!° 
He pointed out that a solution of gelatin which contains a trace of 
acid will precipitate arsenious sulfide sol, while, when a negative 
charge is conferred upon the gelatin by addition of a very small 
amount of ammonium hydroxide, it will then mutually precipi- 
tate with ferric oxide sol. If, however, a slightly positive gelatin 
is mixed with the ferric oxide sol, protection takes place. The 
complex is not precipitated by the addition of a slight amount of 
ammonium hydroxide, but the sign of the charge of the complex 
is changed from positive to negative. 

In view of the modern chemistry of protein solutions, and the 
envelope theory of protection enunciated by Bechhold,*® an 
explanation is available. When a gelatin solution is acidified 
with hydrochloric acid, for example, the gelatin combines with 


37 Z. physik. Chem., 89 (1915), 306. 
38 Archiv fiir Hygiene, 55 (1906), 361. 
39 Biochem. Z., 41 (1912), 461. 

40 Z, phystk. Chem., 48 (1904), 385, 


348 COLLOIDAL BEHAVIOR 


the acid to form the salt, gelatin chloride, which is ionized into 
gelatin cations and chloride anions, 7.e., the gelatin particles are 
positively charged. Hence, when this sol is added to the nega- 
tively charged arsenious sulfide sol, we have a case similar to the 
mutual reaction of ferric oxide sol and antimony sulfide sol 
described early in this chapter. At or near the relative propor- 
tions of gelatin and arsenious sulfide sols, where the “charges 
exactly neutralize each other,’’ there will be mutual precipitation. 
In the presence of a large excess of either the sulfide sol or of 
gelatin there will be no coagulation. Since gelatin is amphoteric, 
it shows a similar behavior toward ferric oxide hydrosol. Addi- 
tion of ammonium hydroxide to a gelatin solution results in the 
formation of ammonium gelatinate. Consequently, there will 
be a range in relative proportions of gelatin (now negatively 
charged) and of ferric oxide sol where mutual precipitation will 
take place, and in the cases of a large excess of either iron oxide 
or of gelatin there will be no precipitation. Certain colloid 
chemists do not favor the idea of the formation of salts by gelatin, 
but they will admit that gelatin becomes positively charged 
in acid and negatively charged in alkaline solutions. 

The writer has used the generally accepted language in dis- 
cussing the mutual precipitation of gelatin with arsenious sulfide 
and with ferric oxide sols. He prefers the following which deals 
with the same as simple chemical reactions. The solution of 
gelatin in dilute ammonium hydroxide contains not only ammo- 
nium and gelatinate ions but also ammonium hydroxide and its 
ionization products, e.g., ammonium gelatinate hydrolyzes in 
aqueous solution. The stability of ferric oxide sol is due to the 
ferric chloride, or acetate, as the case may be, that is combined 
with (or adsorbed by) the ferric oxide particles. When these 
two sols are mixed, the ammonium hydroxide and ferric chloride 
or acetate react to form hydrous ferric oxide. If the condition 
of “isoelectric”? proportions of the interacting sols obtains, then 


precipitation ensues, due to the removal of all of the stabilizing © 


or peptizing agent of the ferric oxide sol, and to the fact 
that there is not sufficient gelatin present to “‘protect’’ it, 7.e., 
to form envelopes around the ferric oxide particles and thus 
keep the latter in dispersion through the solution forces of the 
gelatin. It must be noted as well that gelatin, at or near its 





MUTUAL REACTIONS OF COLLOIDS 349 


isoelectric point (the hydrogen ion concentration at which its 
ionization is at a minimum), is much less stable in solution than 
in the presence of acid or alkali, as shown by Jacques Loeb. 

Similarly, for an arsenious sulfide sol the hydrochloric acid of the 
acidified gelatin solution will drive back the ionization and force 
out of solution the hydrogen sulfide which is the stabilizing or 
peptizing agent of the arsenious sulfide particles. 

On the other hand, when the inorganic colloid is present in 
large excess, the neutralization or the removal of a part of its 
stabilizing agent is not sufficient to throw it out of solution, 
while in the case of a large excess of oppositely charged gelatin, 
no precipitation ensues, due to the enveloping of the ‘‘neutral- 
ized”’ inorganic colloid particles by gelatin, which, by reason of 
its solution force, maintains the ‘‘neutralized”’ particles in sus- 
pension. The sign of the charge depends simply upon whether 
gelatin cations or anions are present, 7.¢., whether it is an acidic 
or alkaline solution. 

Hence when Billitzer mixed gelatin, ammonium hydroxide, 
and ferric oxide sol, in the order named, it is easily seen why he 
got flocculation. When he mixed acidified gelatin, ferric oxide sol, 
and ammonium hydroxide he did not get flocculation of the 
mixture because gelatin films had formed around the ferric oxide 
particles. Addition of ammonia merely changed the envelopes 
of cationic gelatin to gelatin anions. Had he, however, added the 
base slowly, he would have noted a point of very low stability 
of the gelatin-enveloped ferric oxide particles, namely at pH 
4,7, the isoelectric point of this protein. 

The sensitizing action of well-dialyzed albumin (Brossa and 
Freundlich) upon ferric oxide hydrosol can be explained similarly, 
since in neutral aqueous solution this protein is on the alkaline 
side of its isoelectric point, 2.e., it is negatively charged (anionic) 
and forms salts with the ferric ion of the stabilizing ferric salt, or 
causes hydrolysis of the latter, due to its combination with the 
hydrochloric or acetic acid in hydrolytic equilibrium with the 
stabilizing ferric salt of the ferric oxide hydrosol. 

To summarize, a hydrophilic colloid will protect a less stable 
dispersion at all concentrations of the former, provided its sign 
of charge is like that of the latter. If it carries a charge of oppo- 
site sign, it will protect the less stable dispersion, if an amount in 


300 COLLOIDAL BEHAVIOR 


excess of the isoelectric mixture is present. If added in amounts 
such as to give an isoelectric mixture, or less than the same, then 
the stability of the less stable dispersion will be decreased, 
possibly resulting in precipitation. 

Various hydrophilic colloids show different protective effects. 
Zsigmondy*! devised the “gold number” method as a means of 
defining the protective power of a given protective colloid. 
The “gold number”’ of a protective colloid is defined by Zsig- 
mondy as the number of milligrams of the protective colloid 
which just fails to prevent the change of color of 10 cc. of red 
gold hydrosol to blue upon the addition of 1 ec. of a 10 per cent 
sodium chloride solution. 

Zsigmondy prescribes the use of a gold hydrosol that shows a 
weak brownish opalescence to reflected light and a clear bright 
red color to transmitted light. It must not show even a trace of 
violet or blue. Such a sol was prepared by him in reducing gold 
chloride with formaldehyde.‘? A similar sol may be prepared by 
the Bredig are method. 

Zsigmondy’s technique is as follows: Into three beakers, 0.01, 
0.1, and 1 cc. of the protective colloid solution are pipetted and 10 
ce. of gold hydrosol are added to each, followed by vigorous 
shaking for 3 minutes. Then 1 cc. of 10 per cent sodium chloride 
solution is run into each beaker while stirring. Assuming that 
change of color takes place in the first beaker and notin the others, 
the “gold number” is between the values represented by the 
amounts of protective colloid in 0.01 and 0.1 cc. For a more 
exact determination, the procedure is repeated on amounts 
between these limits. The number of milligrams of protective 
colloid which just fails to prevent the change in color of the red 
gold sol to violet is calculated as the ‘‘gold number.” 

The very different protective powers of the hydrophiles are 
seen in Table XVIII. In addition to Zsigmondy’s value,*? 
recent determinations by Gortner*! are given. 


41 Z. anal. Chem., 40 (1901), 697. 

42 Tiebig’s Mander 301 (1898), 30; Zs1gmonpy-Sprar: ‘Chemistry of 
Colloids,”’? John Wiley & Sons, Inc., New York, 1917, p. 90. 

+n, EN Chem., 40 (1901), 697; Terentia pp. 107, 212. 

44 J, Am. Chem. Rake 42 (1920), 595, 


MUTUAL REACTIONS OF COLLOIDS dol 

















TaBLE XVIII 
Gold number 
Colloid 

Zsigmondy Gortner 
Re ee rhe 0.005-0.01 0.005-—0.0125 
SECO Sg 0.01 —0.02 
I a See 0.01 
NSS DOL St a ren 0.08-0.10 
PErOralpioie OCid oc 0.03 —0.08 (Na salt) | 0.15-0.20 
Bayealbiniewcid **.....4....:....) 0.02 —0.60 (Na salt) | 0.10-0.125 
Mn ES ee ti es nace 0.15 -0.5 0.10-0.125 
Beet UUCANI Tee yey ia ee es About 2 
[0 SEEGITES” 0 oe) Os i 6-20 
Meer etree Se OUND) 5 orc | ee ee ee we ee ee 125-150 
REN eRe: re PS etek vhs Lua ees 10—- 15 
PPR ARLALOD ge fpisce fa; vie ns 0 0s > About 25 | 
LTS CES al 0.4-1 a deh 





* These are protein degradation products so named by Paal (Ber., 35 (1902), 2195), 
prepared by heating egg albumin in alkaline solution and precipitating by acetic acid. 
He uses these products in the preparation of protected metallic dispersions. 


These numbers are useful as rough indices of relative protective 
powers only. Probably the concentration and degree of dis- 
persion of the gold sol influence the result. The pH of the solu- 
tion used certainly will affect the values. If the protective colloid 
solution is slightly acid, it will show a poorer protective action 
than one which is neutral or slightly alkaline. One sample 
of gelatin tested by the writer precipitated the gold sol. 

The protective effect is not instantaneous. Some time must 
elapse after mixing for the optimum effect. Three minutes’ 
time is usually sufficient. Dilution is also a factor. In a certain 
case, Zsigmondy found that 0.015 mg. of gelatin in 23 cc. of water 
did not protect 10 cc. of a gold hydrosol, but, when added in 3 ce. 
volume and then diluted with 20 ce. of water, it did protect the 
sol. Apparently, when protection has taken place, dilution does 
not affect it. 


do2 COLLOIDAL BEHAVIOR 


The degree of dispersion of the protecting colloid also affects 
its gold number as shown for gelatin by Menz** and by Elliott 
and Sheppard.‘® This is shown clearly by the experiments 
of the latter. Solutions of gelatin were prepared as follows: 

1. By making up the solutions directly without subsequent 
dilution, as 1g. gelatin to 100 cc. solution for a 1 per cent solution, 
to be heated for 4 hours at 50°C. to establish equilibrium, and 
cooled in a water bath at 20°, at which temperature all gold 
numbers were determined. 

2. By making an original solution of 1 per cent, heating at 
50° for 4 hours, cooling, and diluting to 0.01 and 0.001 per cent 
at 20°. 

3. By making the original solution of 1 per cent at 50°, heating 
for 4 hours, and making further dilutions of 0.01 and 0.001 per 
cent at 50°, with a further 2-hour heating to equilibrium and 
cooling at 20°. 


TaBLE XIX 
ng 
| 
Strength of solution, Gold number 
per cent 
—.koo  IIII— I 
Original 


0.15 
1 0.02 
001 about 0.015 


$e eee 
Diluted at 50° 


0.01 | 0.0075 
0.001 | 0.02 


eee 
Diluted at 20° 
—_ eee 


0.01 0.0075 
0.001 0.02 


*°Z. physik. Chem., 66 (1909), 129. 
* J. Ind. Eng. Chem., 13 (1921), 699. 


MUTUAL REACTIONS OF COLLOIDS 308 


The results shown in the table clearly indicate that the gold 
number decreases with decreasing concentration, that is, the 
protective action of the gelatin increases with decreasing concen- 
trations. This is in agreement with the work of Menz. The 
protective action is not increased by a decrease in the quantity 
of gelatin, but, as the concentration is lowered, the state of divi- 
sion of the gelatin present is altered. At high concentrations 
there is a majority of large particles with some smaller particles 
also; at low concentrations, a majority of very fine particles and 
very few of the larger particles. The larger jelly particles exert 
very little, if any, protective action. It is evident that gelatin 
must be completely in solution to show its maximum protective 
effect. 

Elliott and Sheppard also found that the gold number of 
gelatin solutions increases upon standing, which is concomitant 
with decrease in degree of dispersion of the gelatin. After 
determining the gold numbers of 17 different gelatins of all 
grades and methods of manufacture, they conclude that this 
method is of little or no value in the grading of gelatins. The 
gold numbers differed but little and the classification thus made 
possible was too rough, bearing no simple relation to those prop- 
erties which are of chief interest to users of gelatins. 

Heubner and Jacobs* have tried to determine the gold numbers 
of purified blood proteins (albumin, globulin, and hemoglobin), 
but found that the gold number of a given protein varied with 
the method of preparation. Some of their samples caused the 
gold sol to turn violet in color. This was undoubtedly caused by 
a lack of pH control. Reitstotter*® claims that the gold numbers 
of the various fractions of sera from a number of animals, both 
normal and diseased, are characteristic in most cases, the patho- 
logical condition of the animal influencing the same. It is inter- 
esting to note that he finds that the relative protective action, 
expressed as gold number, is altered by the acidity of the medium. 

The gold numbers of a series of protein degradation products, 
such as proteoses, peptones, etc., have been determined by 
Zunz.*® Attempts have been made to apply the gold number 


47 Biochem. Z., 58 (1914), 352. 

48 Qesterr. Chem. Zig., 25 (1922), 29 through Chem. Abs., 17 (1923), 290. 

49 Archives internat. de Physiol., 1 (1904), 427; 5 (1907), 111, 245; Bull. 
Soc. Roy. des Sci. med. et nat., 64 (1906), 187; Zstamonpy-SpEaRr, pp. 108- 
109. 


304 COLLOIDAL BEHAVIOR 


method to analysis of urines. The presence of protective sub- 
stances in urines have been found,*® but it is doubtful whether 
the method can have any diagnostic value, for reasons already 
shown in other instances. Furthermore, Ottenstein®! has been 
unable to find characteristic gold numbers in urines from certain 
pathological cases. He finds that the gold number of the well- 
dialyzed solids of normal urines range from 3.5 to 7.0, while in 
disease, fluctuating values are found both above and below the 
normal values and not at all characteristic for any one patho- 
logical condition. ‘ 

Protective colloids inhibit the decomposition of hydrogen 
peroxide by platinum hydrosol. Groh’s determinations®? of 
the effect of gelatin, gum arabic, and dextrin are shown in Table 
XX. The time for 50 per cent decomposition of a given amount 
of hydrogen peroxide by a fixed quantity of platinum hydrosol 
was determined both in the absence and presence of varying 
amounts of protective colloid by means of permanganate titra- 
tions of samples of the mixture withdrawn at given intervals. 

















TABLE XX 
rg 
Time for Time for 
Protective colloid a7 pe Protective colloid ve Hees ts 
position, position, 
minutes es minutes 
Nonese 2 Foe eee 20 
Gelating 1.22 4: 265 Gelatin..... 103 
0.1% 4 Gum arabic... 86 0.001 % ; Gum arabic. 21 
Deéexsirine 2) 66 Dextrin..... on 
Gelatin) 150 
0.01% ; Gum arabic. 39 0.0001 % gelatin..... 71 
Dextrin-. | 28 | 











°° Licutwitz and Rosenpacu: Z. physiol. Chem., 61 (1909), 112; Licut- 
witz: [bid., 64 (1910), 144; Sarkowsky: Berlin klin. Wochenschr (1905) 
(through Zst@MoNDY-SPBAR, p. 111). 

51 Biochem. Z., 128 (1922), 382. 

2 Z. physik. Chem., 88 (1914), 414. 


MUTUAL REACTIONS OF COLLOIDS 300 


The order of effectiveness in inhibition of the catalysis is seen 
to be the same as that of protective powers shown by the Zsig- 
mondy gold numbers. Confirmation of Groh’s results is found 
in a recent paper by Iredale®? where the “inhibition number,” 
2.e., that percentage of protective colloid which is just insufficient 
to inhibit the catalytic action of colloidal platinum upon hydro- 


gen peroxide, is found to run parallel to the gold number (Table 
XXI). 
































TABLE XXI 
| | Gold | Ton ee on 
Colloid number number 
number number : : 
ratios ratios 
Beale ae ses: 0.02 Pie Mai 100. 100. 
RP IIS ie ts . = ee ix 104 20. 20. 
PORTIS are ee ee tess. 3 296, 107% 0.66 is 
TELE, Bl 5 Gxc1t)-* 0.40 0.33 
TABLE XXII 
Series I Series IT 
Protective colloid 5 ea Rc meet a 
k Ratio k Ratio 
Le 2 rn 0.055 | 1.00 0.025 1.00 
UGH oe 0.0059 Ori 0.0044 0.18 
BOE ee th» ea 0.0072 0.13 0.0056 D222 
ee ERD SUNT SG Ye a 0.0094 Quel is 0.007 0.28 
op a Po ee 0.035 0.64 0.020 0.80 
RUPE hee, wei in Ses ae 0.083 1.00 0.0185 1.00 
foun tragacanth...'....4:.... 0.028 0.34 0.013 0.72 
Poe -albumiti.......... Pe od he Pee es aes 0.0057 O sae 
LOOT Oso 0.043 0.52 
2 Ra PES. 2 2 Se rr 0.031 0.37 
lt a in tele 0.041 0.50 





53 J. Chem. Soc., 121 (1922), 1536. 


306 COLLOIDAL BEHAVIOR 


A summary of a series of measurements made by Iredale*4 
upon the decomposition of hydrogen peroxide (Z) by platinum 


hydrosol (Bredig) at 25°C. is given in Table XXII. The results 
are expressed in terms of k, the monomolecular reaction velocity 
constant. In ‘Series I,” 0.01 per cent and, in “Series 11,” 
0.001 per cent of protective colloid was present. The mixture 
of platinum sol and protective colloid was always allowed to 
stand 15 minutes before adding to the hydrogen peroxide. 

The effect of varying concentrations of gelatin upon the 


activity of the platinum hydrosol in a mixture containing 30.000 


gram atoms of platinum per liter of mixture is given in Table 
XXIII. Iredale explains the inhibitory effects of the protective 


TaBLE XXIII 
GELATIN, PER CENT k 

None 0.0151 
0.005 0.0027 
0.001 0.0031 
0.0001 ‘ 0.0043 
0.00005 0.0050 
0.00001 0.0107 
0.000005 0.0140 
0.000001 0.0151 


colloids ‘‘on the ground of. selective adsorption resulting in a 
decreased concentration of hydrogen peroxide at the platinum 
surface . . .”’ In other words, he attributes it to the formation 
of films of protective colloid about the platinum particles, in 
accordance with the envelope theory of protective action sug- 
gested by Bechhold. 

The envelope theory of protective action has been definitely 
proved by Jacques Loeb®® by a comparison of the stability 
of protein solutions with that of dispersions of protein-coated 
collodion particles. He prepared collodion suspensions by dis- 
solving dried collodion in pure acetone, adding water to appear- 
ance of turbidity, and distilling off the acetone under reduced 
pressure, whereupon a creamy suspension of collodion particles 


54 J. Chem. Soc., 119 (1921), 109. 
% J. Gen. Physiol., 5 (1922-23), 479. 





er 


MUTUAL REACTIONS OF COLLOIDS ool 


was obtained.*® The preparation of protein-coated collodion 
particles was suggested by his previous experience with collodion 
membranes,*”’ where he found that: 


When collodion membranes are filled with a 1 per cent solution of 
a protein, such as gelatin, crystalline egg albumin, casein, or oxy- 
hemoglobin, there is formed overnight inside the membrane a durable 
film of solid protein which cannot be washed away, even if the interior 
is rinsed out as often as ten or twenty times with warmwater. This 
film betrays itself by its color in the case of oxyhemoglobin. The 
forces which make the film adhere to the collodion must be very strong, 
but they do not depend upon the ionization of the protein, since the 
films are formed no matter whether the protein is at the isoelectric 
point, or whether it is on the alkaline or on the acid side of the iso- 
electric point. .The forces which cause the film formation must be 
those forces of secondary valency responsible for phenomena of adhe- 
sion and cohesion in general.5§ 


Loeb allowed a small quantity of collodion suspension to remain 
overnight in an aqueous solution of a protein. The next morning 
the particles were centrifuged from the protein solution and made 
up to acreamy suspension in water at a desired pH. This suspen- 
sion of protein-coated particles was added to various salt solu- 
tions to note the behavior. The effects of various salts were 
followed by electrophoresis measurements and observation of 
the concentrations of a given electrolyte which caused precipita- 
tion. It was found that the conduct of the protein-coated 
particles is identical with that of a solution of the protein. The 
concentrations of different salts required to precipitate suspen- 
sions of gelatin-coated collodion particles in water are practically 
identical with the concentrations of the same salts required to 
‘salt out”? gelatin from aqueous solutions. Furthermore, Loeb 
found that just as the solubility of gelatin at its isoelectric point 
(pH 4.7) is increased by the addition of certain kinds and 
amounts of salts, soare gelatin-coated collodion particles rendered 
more stable when protected by isoelectric gelatin. 


56 J. Gen. Physiol., 5 (1922-23), 109. 

87 Tbid., 2 (1919-20), 577. 

58 This conduct is like that of gold foil in gelatin solutions observed by 
Zsigmondy as early as 1900 (Zstamonpy-SpEar, p. 112). Gold foil covered 
itself with a film of gelatin that could not be removed by boiling water. 
This layer prevented the amalgamation of the gold with mercury. 


308 COLLOIDAL BEHAVIOR 


Loeb noted a peculiar behavior in the case of egg albumin. 
He found that it is not a good protective colloid for collodion 
suspensions. Investigation of the properties of albumin-coated 
collodion particles showed them to be practically identical in 
stability to that of suspensions of denatured (heat-coagulated) 
albumin particles. He believed that when egg albumin forms a 
film of its solution around collodion particles, the albumin mole- 
cule undergoes a rearrangement or orientation to render its 
water-soluble groups ineffective. He recalled the observation of 
Ramsden®*? on the films of certain proteins which form in aqueous 
solutions, due to the lowering of the surface tension of water. 
Ramsden said that some of these films undergo irreversible coagu- 
lation. It is likewise well to recall that mechanical grinding of 
a dry powder of soluble blood albumin renders the albumin 
insoluble. 

The writer would point out that deposition of albumin at an 
interface as a result of its lowering the interfacial tension does 
not always result in irreversible coagulation or denaturing. 
When an aqueous solution of albumin is shaken with chloroform, 
Ramsden’s so-called ‘‘mechanical coagulation’? appears, 2.e., 
solid films of albumin form at the water-chloroform interfaces 
and settle out. Nolf*! finds that the albumin is chemically 
unaltered in this instance. 

Loeb also found casein and edestin to be poor protectors for 
collodion particles. He defines protective colloids as follows: 

Protective colloids must be capable of forming a durable film on 
the surface of suspended particles and the molecules constituting 
the film must have a higher attraction for the molecules of the solvent 
than for each other; in other words, they must possess true solubility. 
Only in this case can they prevent the precipitating action of low 
concentrations of electrolytes on particles which are kept in suspension 
solely by the high potentials of an electrical double layer. ‘Thus, 
gelatin films, in which the attraction of the molecules for water is 
preserved, have a general protective action, while crystalline egg 
albumin, casein, and edestin, which seem to lose their attraction for 

59 Proc. Roy. Soc. (London), 72 (1903), 156. 

60 HERZFELD and KuincER: Biochem. Z., 78 (1917), 349; WiEcHOWSKI: 
Ibid., 81 (1917), 278 (through Lors: loc. cit.). 


61 Réunion soc. belg. biol. (1921), 273 (through Chem. Abs., 16 (1922), 
2874. 


+ 


MUTUAL REACTIONS OF COLLOIDS 309 


water when forming a film, have a protective action only under limited 
conditions. 

Beans and Beaver®? have performed an experiment which shows 
that the protection of colloidal gold by gelatin is due to adsorp- 
tion of the gelatin by the gold particles. They found that the 
gold particles of a red gold hydrosol (Bredig) were completely 
precipitated by centrifuging for 3 minutes at a force equivalent 
to 32,000 times gravity. The precipitate was black and irre- 
versible. Centrifuging a mixture of 5 cc. of a 0.1 per cent gelatin 
solution and 50 cc. of the gold sol resulted in deposition of the 
gold particles in 16 minutes, but the precipitate was red in this 
instance and could be redispersed to a red sol upon shaking 
with water. On heating some of this precipitate it showed a 
slight charring, indicating the presence of gelatin. The same 
concentration of pure gelatin showed no precipitation of gelatin 
even after 30 minutes’ centrifuging at 32,000 ‘‘times gravity.” 

The envelope or adsorption theory of protection seems to be 
fairly well established. Rideal,®* however, states, as a result of 
hydrogenation experiments, that the protective colloid peptizes 
(z.e., disintegrates and increases degree of dispersion) the metallic 
sol particles, which confirms Bancroft’s hypothesis.** In the 
hydrogenation of phenyl propiolic acid, utilizing both platinum 
and palladium hydrosols protected by gum arabic, as catalysts, 
Rideal found that the protected sols showed greater activity 
than the unprotected ones. There was an optimum amount of 
gum arabic beyond which the activity fell below that of the unpro- 
tected sols. Rideal ascribes this to increase in the specific 
surface of the colloidal metal particles due to ‘‘peptization”’ 
by the gum. Examination of his paper reveals the fact that 
the sols were prepared by adding sodium carbonate to platinum 
or palladium chloride, then gum arabic, and, finally, reducing 
with hydroxylamine. Apparently, the fact that the gum 
might act otherwise than as a ‘‘peptizing’”’ agent seems to have 
been ignored as well as the effect of the reaction products other 
than the colloidal metal. It might be mentioned here that Pearce 
and O’Leary® find that gum arabic inhibits the hydrolysis of 

62 Braver, D. J.: Dissertation, Columbia University, 1921. 

63 J. Am. Chem. Soc., 42 (1920), 749. 


64 J, Phys. Chem., 20 (1916), 85. 
6 J, Phys. Chem., 28 (1924), 51. 


360 COLLOIDAL BEHAVIOR 


methyl acetate. They ascribe this to the adsorption of the 
catalyst, hydrochloric acid, by the gum as shown by pH 
determinations. 

The majority of the investigations on protective colloids have 
been made upon aqueous dispersions, as is to be expected, but 
it is well to bear in mind that their usefulness is not restricted to 
water solutions. Bancroft®® points out that aniline dyes which 
are insoluble in benzene can be dispersed therein by the aid of a 
benzenophilic colloid, such as zinc or magnesium resinate, and 
thus be used in lacquers. 

A practical use of protective colloids is shown by Wegelin,®? 
who found that the adhesion of metallic particles:in grinding 
could be overcome by the presence of an aqueous gelatin solution. 

Protective colloids have been found useful in the manufacture 
of ice cream, their presence preventing the formation of large ice 
crystals and thus insuring smoothness. They have been errone- 
ously called ‘‘fillers”’ in this application. Gelatin, egg albumin, 
and karaya gum are the most popular colloids for this purpose. 
A discussion of the effect of gelatin in ice cream is given by 
Alexander.®* The latter has also observed that the presence of 
protective colloids in milk prevents the formation of lumpy curd 
when the milk is acidified. ® . 

The presence of protective colloids is avoided in analytical 
chemistry, since it has been known, long before they were recog- 
nized as such, that their presence prevented the precipita- 
tion of insoluble compounds. For example, Pauli and Samec”? 
have found that a number of insoluble compounds are more 
soluble in protein solutions than in water. An enumeration of 
the many instances reported would not serve any useful purpose 
here. 

Before closing, it should be noted that protective action, 7.e., 
stabilizing rather unstable particles in solution, is not limited to 
the proteins, gums, soaps, etc. Inorganic dispersions frequently 


66 J. Phys. Chem., 24 (1920), 21. 

87 Kolloid-Z., 14 (1914), 65. 

88 Kolloid-Z., 5 (1909), 101. 

°° [bid., 6 (1910), 197; ALexaNDER and Buttowa: J. Am. Med. Assoc., 
55 (1910), 1196; Archives of Pediatrics (1910), 17. 

79 Biochem. Z,, 17 (1909), 235. 





MUTUAL REACTIONS OF COLLOIDS 361 


act as stabilizing agents, but the writer is not so confident that 
in such cases, as noted below, the action is due to film formation. 

According to Bancroft, hydrous chromium oxide adsorbs the 
hydrous oxides of iron, nickel, cobalt, manganese, and copper 
and, consequently, protects them to a certain extent, making 
them apparently soluble in potassium hydroxide solution. When 
the chromium salt is in large excess relatively to the other salt 
present, none of the other hydrous oxide is precipitated when 
not too great an excess of alkali is added, but, when the other 
salt is in excess, everything is precipitated upon addition of 
alkali, chromium oxide being adsorbed and carried down in the 
precipitate. Bancroft refers to the protective action of the 
uranyl salt of molybdic acid. ‘Tungstates are precipitated by 
uranyl salts while molybdates are not. If a uranyl salt is added 
to a solution of a molybdate and tungstate, nothing is precipitated 
if the molybdate is in excess, while practically all the molybdate 
is carried down when the tungstate is present in excess. 

In the tables showing the mutual precipitation of inorganic 
colloids early in this chapter, it was seen that when a small 
amount of one colloid was mixed with a large amount of another, 
no precipitation took place. The former lost its stabilizing agent, 
to be sure, through chemical action with that of the latter 
(it was neutralized), but remained dispersed, due to combination 
with the particles of the latter. 


CHAPTER XIV 
ENZYMES 


By 


EK. FRANKLAND ARMSTRONG 


Enzyme action, in reality, is an interaction in which water is 
either distributed upon a single molecule, which is thereby 
resolved into two others 


A.O.B + H.OH = A.OH + B.OH 


or is divided between two molecules in such manner that, while 
the one is hydroxylated, the other is hydrogenated 


A + 2H.OH +B = A(OH), + B.He 


The study of enzyme action is thus, at bottom, a study of water, 
to the chemist the most elusive of all substances. Enzymes 
themselves are part of a larger colloid complex and the actions 
in which they take part are all actions at a surface, as distinct 
from action between substances like acid and alkali in true solu- 
tion. The investigation of surface action is thus the primary 
task if we are to understand enzymes. 

Enzymes are regarded as catalysts, but we are still largely 
in the dark as to their real nature; much is known as to the 
extent and manner of their action, and it has been customary to 
think of them as definite chemical entities, though it is probable 
that the catalytic activities associated with them are to be 
connected with definite aggregates of groups in a larger molecule, 
with the consequence that the enzyme, as such, is incapable of 
existing. Their activity in the main is hydrolytic, that is, they 
render water molecules active. We define a catalyst as the 
agent which brings about the inclusion of the interacting sub- 
stances in a circuit within which change takes place as soon as 

362 


ENZYMES 363 


the circuit is established; it may also be the actual agent by which 
the change is effected. 

Enzymes are present in animal and vegetable tissues, from 
which they are obtained in a concentrated rather than a pure 
condition by a variety of methods involving, in the first place, 
the rupture of the cell wall, and then sometimes the decomposi- 
tion of a larger complex to liberate the active enzyme, such as is 
best effected by self-digestion or autolysis. The enzyme is 
precipitated by cautious addition of alcohol or acetone, redis- 
tributed in a little water, and the precipitation repeated, if 
desired. Without going into details well known to workers in 
these fields, all the practical methods are such as avoid drastic 
treatment likely to destroy colloid aggregates or, in a word, affect 
surface, which the writer regards as the prime essential for an 
active enzyme. It will be gathered from the above that enzymes 
are relatively unstable and less active under laboratory condi- 
tions, and the critically minded will treat with caution results 
obtained with too highly purified, that is, overtreated, products 
- from the point of view of the explanation of their behavior as 
colloids. | 

Enzymes are active in liquids which have been filtered— 
hence, the term “soluble ferments”’ long applied to them—but 
they are also active in the insoluble state both in aqueous solu- 
tions and other media. ‘They are non-diffusible through parch- 
ment paper. The outstanding property of enzymes which 
distinguishes them from all other catalytic agents is their specifi- 
cally selective nature; any explanation of their behavior must 
take this into account. Indeed, far from being exaggerated, as 
stated by Bayliss in his British Association report (1918), this 
selective and limited action is one of the outstanding factors in 
the regulation of metabolism in living matter and its importance 
has been imperfectly understood by many writers on enzyme 
action. | 

In general, catalysts become more and more active as the 
extent of their surface is increased. A lump of metallic nickel, for 
example, is almost inactive in promoting reduction, but particles 
obtained by abrasion become more active as their size diminishes. 
Metal in the very finely divided particulate or colloid state, as it 
is termed, is very active, and a still finer state of division and 


364 COLLOIDAL BEHAVIOR 


greatest activity is obtained by precipitating nickel from a dilute 
solution of its nitrate on the surface of an inert carrier, such as 
kieselguhr, and afterwards reducing the oxide so formed at a 
suitable low temperature to metallic nickel. One gains the 
mental picture of a film made up of extremely small particles 
extending over the surface of the interstices of a sponge-like carrier 
so that each particle is able to come into actual contact with gas 
or liquid. Loss of activity is observed when the temperature 
of preparation has proved to be too high, thereby causing the 
particles to coalesce and reduce the amount of surface. 

Now, enzymes are essentially particulate colloids in an even 
finer state of division or, more correctly, dependent even more on 
surface conditions than the active metal catalysts. Theaccumu- 
lated knowledge of the methods of preparation of an active 
enzyme, the care which is necessary with regard to temperature, 
time of extraction, reaction of the medium, presence of inorganic 
salts and of various poisons affords very definite evidence that in 
chemical phraseology the enzyme is very unstable or, in other 
words, its active surface must not be impaired. Enzymes are 
not used on supports like metals, but the large molecules of which 
they are only a section themselves act in this fashion. The 
pseudosoluble colloid enzymes are not in a state of true dissolu- 
tion but are able to develop the maximum surface area, probably 
even more so than the most active metallic catalysts and 
certainly infinitely more than other inorganic catalysts which act 
as hydrolytic agents. This fact offers a ready explanation of the 
phenomenal activity of enzymes compared with other chemical 
agents under like conditions of temperature. 

The action of the majority of the really authenticated enzymes 
is hydrolytic; the following classes have been studied: 

Saccharo-clasts, such as diastase, invertase, maltase and 
lactase, which act on starch and the dissaccharides, and emulsin, 
which attacks many natural and artificial glucosides. 

Lipoclasts, which split fats into glycerol and fatty acids. 

Proteoclasts, which break down proteins and polypeptides 
into their constituent amino acids. 

Urease, which hydrolyzes urea to ammonia and carbon dioxide. 

Oxidase, reductase, catalase, concerned in biological oxidations 
and reductions... 


tren he) 


ENZYMES 365 


An examination of each of these processes follows in such detail 
only as will serve to emphasize certain of their general and 
specific behavior as colloids. 


SACCHARO-CLASTS 


The literature relating to these is so extensive as to have lost its 
value unless very critically studied. Much of the work recorded 
has been done without a full understanding of the necessary 
technique, so that in many cases the measurements recorded are 
those of the influence of secondary disturbing factors and not of 
the enzyme itself. Two facts have been definitely established, 
the one that the action of the enzyme is absolutely selective, one 
enzyme acting on 6-glucosides alone, another on 6-galactosides 
only, and a third on a-glucosides only, while invertase acts only on 
cane sugar. The selective action is connected only with the 
sugar part of the molecule, the effect of the non-sugar radicle 
being merely quantitative and secondary. The second fact is 
that all the observed behavior of these enzymes, when studied 
quantitatively, is in agreement with the idea that action takes 
place at a surface and that under ideal conditions, with a regular 
access of substrate and regular removal of products and the 
elimination of all poisons, action takes place at a steady rate in 
conformity with the hypothesis that it is preceded by the forma- 
tion of an additive complex of enzyme and sugar. 

Equal amounts of substance are hydrolyzed in successive equal 
intervals of time, that is to say, the course of action is expressed 
graphically by a straight line. On the contrary, when cane 
sugar is hydrolyzed by acid, the action is in accord with the laws 
of mass action and is expressed by a simple logarithmic curve. 
Numerous workers have sought to force mass action laws into 
use in interpreting enzymic action, believing that the analogy 
between the acid catalyst and the supposed soluble enzyme cata- 
lyst was complete, and ignoring the great difference between the 
character of the two actions. It is desirable to emphasize, there- 
fore, that the more nearly the proper effect of the enzyme is 
understood the more it is seen to depart from the mass action 
“laws,” thus showing that it is not uniformly distributed but 
itself a focus of attraction and concentration. The rate of 


366 COLLOIDAL BEHAVIOR 


change is definitely linear in cases where the hydrolysis is all but 
complete, as is best exemplified in the case of urease; the rate 
diminishes when there is reversal, as in the case of lipase. The 
ideal conditions demanded to illustrate this hypothesis are often 
difficult to realize in the laboratory, but they undoubtedly obtain 
under natural conditions in the cell. 

It is desirable very briefly to illustrate the selective nature of 
enzymes. As the graphic formulas show, the only difference 


HC—OR «ROE 
| 


COR HCO 
| SO 
Lehi Ove! ae ie 
| 
HO HC 


| | 
HCOH HCOH 


O 


| | 
CH.OH CH.OH 
a glucosides B glucosides 


between a and £ glucosides is in regard to the arrangement of the 
groups attached to the topmost carbon, here shown as placed to 
the right and left of the carbon chain. Yet maltase acts on a 
glucosides alone, emulsin on glucosides alone. By the inter- 
change of the H and OH groups attached to either of the four 
next carbon atoms of the chain, isomeric sugars are obtained, 
€.g., mannose, galactose, gulose, etc. Inno case are the glucoside 
derivatives hydrolyzed by either maltase or emulsin By well- 
known methods the chain of carbons has been shortened or 
lengthened and other sugars and their glucoside derivatives 
prepared. In no case are they acted on by these two enzymes. 
As we have recently learned, some of the changes mentioned 
involve an alteration in the position and magnitude in the oxygen 
ring which may contain three, four, or five carbon atoms in 
addition to oxygen; these changes also render the enzymes incom- 
patible with the glucosides. This subject has been studied 
in great detail, but it is unnecessary to multiply examples; the 
proof of the selective action is absolute. 


ENZYMES 367 
LIPASE 


Lipase, first discovered in the germinating seeds of Ricinus 
communis, is present in many seeds, usually in the germinating 
rather than the resting stage, its function being to make the fats 
stored up in the seed available for the growing embryo. Animal 
lipase is present in most tissues, especially the liver; it appears to 
cling to the solid particles of tissue cells. It is very doubtful if 
active filtered extracts of the enzyme can be obtained; vegetable 
lipase, in particular, is very sparingly soluble in water. The 
experiments of H. E. Armstrong, Connstein, Kastle, and Loeven- 
hart, amongst others, have shown that vegetable lipase acts 
preferentially on natural fats, other ethereal salts being but little 
attacked by it; animal lipase, on the other hand, is quite active 
in hydrolyzing simple esters, but acts on the natural fats with 
difficulty. Such difference is probably more apparent than real, 
and is due to the difficulty of securing a satisfactory emulsion. 

In other words, the activity of the colloid enzyme is dependent 
on proper association with the fatty material; in the liver, lipase 
and fat are in close conjunction, not suspended in water. 

Probably vegetable lipase powder (Tanaka) contains an 
emulsifying constituent on which its activity is in no small 
measure dependent. H. E. Armstrong and Gosney have sug- 
gested that the properties of lipase are to be accounted for on the 
assumption that it is a colloid molecule possessed of a carboxylic 
or even a phosphoric group so situated that it cannot be self- 
neutralized but yet sufficiently near to a basic center to be inter- 
fered with by any acid which can combine with this latter. 
The interaction must be supposed to take place at and between 
surfaces separated at most by a thin, almost molecular, film of 
water. Presumably the rate at which interaction takes place is 
dependent on the conditions at the colloid surface; as these can- 
not be expressed in terms of the concentration of the solution, it 
is impossible to apply the laws of mass action to the interpreta- 
tion of the changes observed. As previously explained, under 
ideal conditions equal amounts of material will be hydrolyzed by 
a given quantity of enzyme in successive equal intervals of time; 
in practice, numerous secondary causes may effect departure 
from this rate. 


368 COLLOIDAL BEHAVIOR 


UREASE 


This enzyme, which is the cause of the alkaline fermentation of 
urea, 1S present in certain microorganisms but was not investi- 
gated to any extent until its discovery in the soybean and in 
various leguminous seeds made a plentiful supply available. 
Its action, like that of other enzymes, is essentially specific, it 
being entirely without action on substituted ureas. The addi- 
tion of ammonia has a retarding effect on the hydrolysis, while 
the presence of carbon dioxide accelerates the activity of the 
enzyme. 

The prevailing view as to the mechanism of action of the 
enzyme, is that, in the first place, there is a combination between 
urease and the substrate followed by disruption of the combina- 
tion with liberation of the urea as ammonia and carbon dioxide. 
The ease with which measurements can be made has rendered 
urease an ideal material for studying the course of enzyme action, 
which has been done by H. E. Armstrong and Horton and 
subsequently by Van Slyke. The work of the former investiga- 
tors amply justifies the belief that enzymic action takes place 
entirely at the surfaces of colloid particles suspended in the 
solution of the hydrolyte and not between substances which are 
all in true solution. This hypothesis hardly differs from the 
original suggestion of H. E. Armstrong and Horton in 1912, 
who regarded urease as a feebly acidic substance uniting with the 
feebly basic substance urea before it can produce change. The 
presence of ammonia, a more basic substance, interferes with 
such union and, consequently, retards change. 

The enzyme solution is prepared from the fat-extracted, ground ~ 
soya meal by digestion with water and simple filtration; any 
attempts to purify the enzyme and to free it from the large 
amount of albuminous matter resulted in less active preparations. 


OXIDASES 


The numerous changes in the living body involving oxidation 
and reduction are attributed to enzymes. Molecular oxygen is 
quite unable to burn the substances which are so easily dealt 
with by the tissues of the body, and yet it is in this form that the 
oxygen reaches them. One conception of oxidases, based on the 


ENZYMES 369 


views of Bach, Bertrand, and Engler, is of a system consisting of 
an organic substance capable of taking up molecular oxygen to 
form a peroxide and parting with one or even both atoms to 
another substance, the transference being determined or accel- 
erated by a peroxidase. In the plant, as Miss Wheldale has 
shown, catechol derivatives are capable of acting in this manner. 

Many biological oxidations take place in aqueous solution in 
the absence of molecular oxygen, provided a reducing substance is 
also present—the oxygen is derived by the splitting of water, 
one substance, termed the ‘‘hydrogen acceptor,” suffering reduc- 
tion, and another, the ‘“‘oxygen acceptor,’”’ being simultaneously 
oxidized. In practical work methylene blue is very commonly 
added as an extraneous hydrogen acceptor, the disappearance of 
color establishing that the oxidative change has taken place. 

Biological oxidation is thus essentially a process of splitting 
water into hydrogen and oxygen. If A is the substance oxidized 
in vitro we have, under anaerobic conditions 


A + 2H.OH + B — A(OH), + BH, 
and under aerobic conditions 
A + 2H.OH + O, — A(OH). + HO.OH — AO, + 2H.0 


A number of substances are oxidized by catalysts derived from 
living tissues under either anaerobic or aerobic conditions, there 
being evidence to show that the same catalyst controls both 
processes. There is, however, an increasing amount of evidence 
that the catalysts are in many cases specific in relation to the 
substrate, and it must, therefore, be assumed that in this case 
also there is some structural relation between enzyme and 
substrate and that the formation of an additive complex is a 
prelude to action. 

The most recent summary of this subject by Hopkins, which 
has been freely drawn upon in making this abstract, emphasizes 
the diverse character of the catalytic systems which control 
biological oxidations. They differ largely in stability, some 
displaying the characters of enzymes in ordinary colloid suspen- 
sion, whereas others are more closely associated with solid struc- 
tures in the cell. In addition, the cell undoubtedly contains 
catalytic agents able to constitute oxidizing systems which are 


370 COLLOIDAL BEHAVIOR 


both more stable and also thermostable and not to be classed as 
enzymes—such are iron compounds. 

Hopkins attaches particular significance to the disulfide and 
thiol groups which in the cell suffer reversible oxidation and 
reduction. A dipeptide, glutathione, present in most actively 
living tissues, discovered by him, consists of glutaminic acid 
condensed with cystein or, in the oxidized form, with cystine. 
The two forms may be represented as follows, G standing for a 
glutaminic acid nucleus attached either to the amino or carboxyl 
groups. 





CH:.SH CH,’ 8: 5: ome 
| 
CH.NH = CH.NH CH.NH 
| | | 
COmG Come COnG 


It is of some physiological significance that in the case of the 
dipeptide both the disulfide and thiol form are freely soluble in 
the tissue fluids, whereas cystine isnot. Hopkins has shown that 
in the living tissues the thiol group of glutathione is autoxidizable, 
while the disulfide group thus formed is under similar conditions 
freely reducible to the thiol form by factors present in the tissues. 


The exact explanation of all the observations made by Hopkins. 


in connection with this reaction is still under investigation 
and the reader is referred to the original literature, but it is 
possible to indicate that compounds, such as glutathione, forming 
part of a polypeptide which, in turn, is part of a larger colloid 
complex, may well represent centers of enzymic activity in such 
molecules. 

As an example of the selective nature of oxidases, the work of 
Morgan, Stewart, and Hopkins may be quoted, showing that an 
enzyme present in the liver tissue and in milk is able, under 
aerobic conditions, in presence of methylene blue as hydrogen 
acceptor, to oxidize hypoxanthin at exactly twice the rate that it 
oxidizes xanthin, the product of oxidation being uric acid in 
each case. Under anaerobic conditions uric acid is produced at 
the same rate from both compounds. Closely related substances, 
such as guanine, caffeine, uracil, thymine, histidine, etc., are not 
oxidizable under these conditions. The oxidation has been shown 


prem ienns, 


ENZYMES ovl 


to proceed at a linear rate, indicating that the nitrogen compounds 
are held at the surface of the enzyme and not released until 
fully oxidized. 

Having illustrated certain features in the behavior of the indi- 
vidual enzymes, it is possible to elaborate with some certainty a 
more general conception of the manner of their action. In doing 
so, however, it is necessary to touch on several subjects of a 
controversial nature. 

In the first place, an enzyme has a double function, that of 
attracting or holding the hydrolyte and that of determining its 
hydrolysis. It acts both as an acceptor and as an agent. Some 
writers would deprive it of the latter function, believing that 
when the hydrolyte is suitably held—forming a part, it may be, 
of an electrolytic circuit—certain other forces come into play or, 
perhaps, the water molecules themselves become active as hydro- 
lytic agents. The difference between the two theories is more 
one of detail than of consequence and the question is one which is 
ripe for immediate further investigation. 

The activity of an enzyme as an acceptor is the basis of our 
hypothesis. In no other way can the highly selective character 
of enzymes, as is especially illustrated in the case of the saccharo- 
clasts, be explained. It is, then, a natural corollary that this 
close relationship between enzyme and hydrolyte involves a 
similarity in structure; enzyme and hydrolyte must contain 
the same groupings, no doubt as part of a much larger molecule. 
It has been suggested, for example, that the glucoside-splitting 
enzymes contain all or part of the glucose skeleton; lactase is, 
perhaps, akin to galactose; invertase may have the whole cane 
sugar structure or that part involving the junction of glucose and 
fructose residues, the exact nature of which still baffles us; lipase 
may contain a carboxylic grouping; urease such a residue as 
arginine. Such conjectures still lack experimental proof. 
The crude enzymes contain such elements as suggested, but 
their purification is at present impossible. At all events, the 
hypothesis involves the assumption that the relationship of the 
acceptor section of the enzyme to the hydrolyte is that of a super- 
‘posable and, therefore, practically identical radical. This 
explanation goes somewhat further than Emil Fischer’s lock-and- 
key conception, which is generally pictured as a close fitting 


312 COLLOIDAL BEHAVIOR 


of the enzyme and substrate molecules, like the pieces of a 
jigsaw puzzle. 

The conception of the enzyme holding the hydrolyte demands 
further consideration as to the nature of the attachment. There 
may either be formed a loose intermediate complex, depending on 
the attraction of similar grouping for one another or on the 
partial valencies exercised by oxygen and nitrogen atoms, or, as 
some would prefer, the hydrolyte is concentrated, absorbed, 
adsorbed, or in some other way held at the surface of the enzymes. 
The nature and behavior of such intermediate complexes is best 
left for consideration in another paragraph. 

The only hydrolytic agents known to us are acids or alkalies, 
so that it is fair to assume that if the enzyme acts as an agent, 
it does so in virtue of the colloid aggregate containing an acid 
radicle so situated with reference to the acceptor section that the 
hydrolyte, when combined or associated with this, is in immediate 
proximity or sufficiently near to enable a conducting circuit to 
be set up. It is an outstanding experimental fact in connection 
with hydrolytic enzymes that they are very sensitive to alkalies : 
it is impossible to be too careful to exclude alkaline impurities 
and all work must be carried out in hard glass utensils, using 
bottles, pipettes, and measuring vessels of the same material. 
For the same reason, the addition of amphoteric substances, such 
as glycine, is often of advantage. 

It is generally accepted that at a colloid surface in water the 
water molecules are in a state of greater activity than the average 
activity of the water in the neighborhood: whereas the normal 
water molecule is a complex (H2O), it is probably the simple 
molecules H,O which are active in chemical change and which, 
therefore, are attached to the surface of the colloid. As a con- 
sequence of this property of the surface, the molecules of hydro- 
lyte are absorbed from the solution and concentrated at the 
surface. If the active section of the enzyme is only a portion 
of the whole colloid aggregate, it probably remains highly charged 
with the hydrolyte almost up to the point at which the supply 
in the solution is exhausted, the rate at which liquid diffusion takes 
place being so great that the supply of hydrolyte to the surface is 
not a limiting factor. A constant supply of hydrolyte is, accord- 
ingly maintained at the surface of the enzyme. 


ENZYMES 303 


Alternatively, on the intermediate complex theory, they are 
attracted to the surface and loosely combined with it. Under 
either hypothesis no state of fixity is imagined. The loose 
intermediate complex is continually being formed and broken 
down again; the absorbed hydrolyte is not permanently held, but 
oscillates between it and the liquid of which the water molecules 
also exercise a pull on the soluble hydrolyte. Consequently, 
only a certain proportion of effective contacts are made; it is the 
number of these in the unit of time which determines the rate of 
hydrolysis. 

Normally, when two substances, such as sugar and acid, act 
on one another in aqueous solution, the solvent water exercises 
an attraction for both which, in the case of acid, is so great that 
only a small fraction of the total present is able to make effective 
contact with the sugar. Hence, the apparent low activity of 
acids as hydrolytic agents. In the case of the colloid enzyme, 
water has very little attraction and, as the colloid is in a finely 
divided state, which is equivalent to the maintenance of the 
largest possible amount of surface, the hydrolyte tends to accu- 
mulate at the surface and the attractive influence of the solvent 
water is largely overcome. That is to say, a relatively large 
proportion of the hydrolyte is brought into effective conjunction 
with the enzyme agent which is placed under specially favorable 
conditions. 

We have assumed elsewhere that the carboxylic radicle is the 
agent; this, though only weak in the majority of acids, has a 
high efficiency in some, as, for example, the substituted acetic 
acids, and it may be especially powerful in the enzymes. 

A good many physical chemists now accept the probability of 
orientation on an inert surface, owing to the affinity of certain 
groups for the water of the liquid phase; for example, in the 
case of a fatty acid film on water, the carboxylic group has a 
greater affinity for the water than the terminal CH; groups and 
the molecules are regarded as arranged in parallel lines with the 
CH; group uppermost, 7.e., farthest away from the surface. 

In the case of enzymes, all the evidence as to the very specific 
nature of their action is strongly in favor of some definite form 
of orientation—for the moment in the case of the saccharo-clasts 
one may imagine the hydrolyte as lying along or fitting against 


374 COLLOIDAL BEHAVIOR 


the surface of the enzyme. Whether such a state of things 
be described as adsorption or the formation of an intermediate — 
complex is immaterial. As Bayliss has recently pointed out, 
adsorption is shown only when a sufficient number of atoms 
are joined to form a surface. While the writer prefers to think 
of the forces involved as chemical, this point is largely immaterial, 
as the work of the Braggs has indicated that, in the case of 
crystals, the forces responsible for cohesion, chemical union, and 
electrical behavior are one and the same. 


INTERMEDIATE COMPLEXES 


The conception of intermediate additive complexes is one 
which presents no difficulty to the organic chemist, who has used 
it to explain many transformations, but apparently it is not so 
obvious to others and it requires, therefore, a little elaboration. 
The chemist is acquainted with types of compounds ranging from 
the very stable, such as methane, to those of only transient 
existence breaking down into other compounds at the moment 
of their formation and being recognizable only by their inter- 
actions, often color changes, or by means of some more stable 
salt or other derivative. It should not be difficult, then, to 
imagine complexes which are being formed and decomposed 
again, the two interactions taking place with nearly the same 
velocity. Whereas there is only one method of forming the com- 
plex, there is, in the cases in question, more than one way of 
decomposing it and, in practice, it will break down in each of the 
possible ways, the amount of each product formed being due to 
other influences. For example, there is evidence that nickel, in 
its active state, and oleic acid form a complex in the presence 
of hydrogen, which, besides breaking down to nickel and oleic 
acid, also yields nickel and elaidic acid, the two acids being 
cis-trans isomerides, and, further, forms one or more isomeric 
oleic acids in which the unsaturated linkage has shifted to another 
position. 

Such intermediate complexes can from their very nature never 
be isolated and characterized and their existence proved, as is 
demanded by some critics; otherwise they would not be suffi- 
ciently unstable to break down immediately to new products. 


ENZYMES 379 


ENZYMES AS SYNTHETIC AGENTS 


From the theoretical point of view, enzymes, like other cataly- 
tic agents, should be capable of inducing synthetic as well as 
analytic changes, and it is believed that much of the synthesis in 
the living cell is effected by their agency. In cases where their 
action is least specific it has been clearly established that, in 
concentrated solutions of substrate, the enzyme promotes syn- 
thetic change, as, for example, lipase in the case of esters of fatty 
acids, and emulsin in the case of alcoholic solutions of sugars, when 
the corresponding alkyl-glucosides are produced up to the point 
of equilibrium, depending on the concentration of the sugar. 
In these as in others, well-authenticated experiment is in accord 
with theory. 

Most interest attaches to the synthesis of the natural sugars 
and here there is much that is obscure. ‘The synthesis of cane 
sugar from a mixture of glucose and fructose certainly does not 
take place in the presence of invertase, and the synthesis of maltose 
from glucose in the presence of maltase has been the subject 
of controversy. An explanation of the subject will probably be 
found in the new discoveries about the unexpected complexity of 
the structure of the sugars. Glucose and fructose in aqueous 
solution, for example, are not present in those structural forms 
_ which are believed to be the units of cane sugar and, unless the 
enzyme can bring them into this form, synthesis would appear to 
be impossible. Similarly, maltase acting on glucose in aqueous 
solution can only make maltose, if the right modification of glu- 
cose is available; otherwise synthesis, if effected, must result in 
the formation of an isomeride of maltose, for which fact there is 
definite experimental evidence. Thus, apparent departures 
from simple theory in the case of enzymes are to be attributed to 
the secondary cause of structural isomerism and not to any 
eccentricity of the enzyme. 


SUMMARY 


The present state of our knowledge of enzymes may now be 
summarized somewhat as follows. The enzyme is an aggregate 
of groups in a much larger colloid complex and not an entity in 
the strict sense of the term. Action takes place at the surface 


376 | COLLOIDAL BEHAVIOR 


of the colloid and involves a momentary association between 
the enzyme and the substance on which it acts. The processes 
of purification of an enzyme involve, on the one hand, the libera- 
tion of the groups at the surface in an active form and, on the 
other, the elimination of all factors which are injurious to surface 
action, including both those which tend to dirty the surface and 
those which, by combining with a portion of the enzyme, render it 
inactive. The highly specific nature of most enzymes makes it 
probable that there is some relation in structure between enzyme 
and the substance on which it acts, as otherwise the additive 
complex could not be formed. In the living organism, this 
highly selective activity forms the mechanism regulating meta- 
bolism; without it, the downgrade changes in life would be 
largely uncontrolled. Once the enzyme complex is formed, an 
electrochemical circuit, in which active water molecules take 
part, is completed and, the necessary energy being supplied, in 
manner which has yet to be explained, the disruptive changes 
take place leaving the enzyme free to form a fresh complex. 


REFERENCES 


ARMSTRONG, E. F. and others: Enzyme action I-XX, Proc. Roy. Soc. 
(1904) onwards, particularly 73 (1904), 500; 76 (1905), 592; 79 (1907), 
360; B, 85 (1912), 363; B, 86 (1913), 561. 

ARMSTRONG, E. F. and Hitpitcu: Catalysis at solid surfaces, Proc. Roy. 
Soc., A, 96 (1919), 137; A, 98 (1920), 27. 

Armstrong, H. E. and Gosnry: Lipase, Proc. Roy. Soc., B, 86 (1913), 586; 
B, 88 (1914), 176. 

ARMSTRONG, H. E. and Horton: Urease, Proc. Roy. Soc., B, 85 (1912), 
109-127; B, 86 (1913), 328-343. 

Bayuiss: British Association Report on Colloid Chemistry (1918), 143. 

BovurQuELoT and BripEu: Synthesis of glucosides with emulsin, Ann. chim. 
phys., 28 (1913), 145. 

Fiscner, E.: Configuration and enzyme activity, Ber., 27 (1894), 2985, 
3479; 28 (1895), 1429. Z. physiol. Chem., 107 (1919), 176. 

Hopkins, F. G.: Oxidases, Biochem. J., 15 (1921), 286; J. Biol. Chem., 54 
(1922), 527. . 

Kast Le and Lorvenuart: Lipase, Am. Chem. J., 24 (1900), 491. 

Moraan, Stewart, and Hopkins: Oxidases, Proc. Roy. Soc., B, 94 (1922), 
109. 

TANAKA: Lipase, Chem. Soc. Abstr., (1910), i, 800. 

Van Stryke and Cutten: Mode of action of urease and of enzymes in 
general, J. Biol. Chem., 19 (1914), 141. 


CA eee. 
JELLIES AND GELATINOUS PRECIPITATES 
By 


Harry B. WEISER 


If certain colloidal solutions of highly hydrous substances are 
caused to coagulate under suitable conditions, a bulky semi- 
solid mass is formed which is known as a jelly-like precipitate 
or a jelly, when there is no supernatant liquid, and as a gelatinous 
precipitate, when a portion of the liquid phase is visible as such. 
Thus, if a small amount of an electrolyte with a multivalent 
anion, such as potassium sulfate, is added to a colloidal solution 
of hydrous chromic oxide, the sol coagulates with the formation 
of a firm, uniform, transparent jelly which encloses all the liquid; 
on the other hand, if an excess of electrolyte is employed, rapid 
agglomeration of the colloidal particles takes place, forming a 
bulky, translucent, gelatinous precipitate which remains sus- 
pended in the liquid phase.! The chromic oxide jelly may be 
converted into a gelatinous precipitate by shaking, thereby 
partially destroying the uniform jelly structure and so permitting 
_ a portion of the enclosed liquid to escape. Jellies and gelatinous 
precipitates are two forms of gels. Gels of the hydrous oxides, 
such as chromic oxide, which lose their elasticity and become 
powdery on drying, are called rigid or non-elastic gels to dis- 
tinguish them from elastic gels, such as gelatin and agar, which 
are characterized by their perfect elasticity through certain 
narrow limits and by retaining their elasticity and coherence on 
drying. 


THE STRUCTURE OF GELS 


Since a working theory of the structure of gels is essential for 
a systematic discussion of the preparation of the two types, it 
seems advisable to take up the question of structure first. This 


1 Weiser: J. Phys. Chem., 26 (1922), 429. 
377 


078 COLLOIDAL BEHAVIOR 


question has doubtless received more attention at the hands of 
skilled investigators than any other single problem in the field of 
colloid chemistry. And it is still a very live problem, since there 
are such fundamental differences of opinion among eminent au- 
thorities as to the nature of a jelly. Thus Robertson, Procter,? 
and Katz’ regard jellies as homogeneous single-phase systems, 
solid solutions or semi-solid solutions ‘“‘of the exterior solution 
in the colloid in which both constituents are within the range of 
the molecular attractions of the mass.”’ Wo. Ostwald‘ considers 
gels to be two-phase liquid-liquid systems possessing an inter- 
facial tension, and a similar view is held by Bancroft. The vast 
majority of investigators, however, incline to the view that 
jellies are two-phase solid-liquid systems, in which there is a 
network or cellular arrangement of solid phase permeated by 
liquid. 

The evidence in support of the solid-solution theory of jelly 
structure has been drawn largely from investigations on the 
swelling of substances. Thus Katz, in an exhaustive mono- 
graph, points out that there is a close similarity in the phenomena 
associated with swelling and in the changes which accompany 
the formation of binary liquid mixtures. This parallelism leads 
to the conclusion that the swelling process is simply the forma- 
tion of a solid solution between water and the swelling substance. 
Similarly, from a study of the system gelatin-acid-water, 
Procter concluded that gelatin combines with acid, forming 
easily dissociated salts, and that the volume of a swollen jelly 
under equilibrium condition is determined by the osmotic 


pressure of the gelatin salts and the Donnan equilibrium. 


While such a view may explain the swelling of gelatin, it seems 
inadequate to account for the change in viscosity and the loss of 
mobility when a warm solution of gelatin, for example, is cooled. 
Procter meets this difficulty by postulating the formation of 
tenuous and possibly flexible crystals which interlace, and possi- 
bly anastomize, when a warm molecular solution “sets” on 


2 J. Chem. Soc., 105 (1914), 313. 

* Kolloidchem. Bethefte, 9 (1971), 1. 

* Pfiiger’s Arch., 109 (1905), 277; 111 (1906), 581; “Theoretical and 
Applied Colloid Chemistry,’’ translated by Fischer, 1917, p. 103. 

® “Applied Colloid Chemistry,” 1921, p. 242. 


JELLIES AND GELATINOUS PRECIPITATES 379 


cooling. ‘These crystals are not of microscopic dimensions and 
the network is, therefore, so fine that both solvent and crystals 
are within the range of each other’s molecular forces. The 
solid-solution theory and the two-phase solid-liquid theory differ, 
therefore, in regard to the size of the particles forming the net- 
work. Since there is no reason to doubt that these particles are 
frequently of microscopic dimensions, the solid-solution theory 
cannot be of general application. 

Ostwald’s theory that jellies are simple emulsions of spherical 
or more or less distorted globules in a liquid medium meets with 
serious objection at the outset, since there are no emulsions known 
that have really the properties of jellies. The inorganic jellies 
certainly could not be looked upon as emulsions, particularly in 
those cases where a rigid crystalline structure has been detected. 
Recalling the applicability of Boltzmann’s gas theory,® which 
considers molecules to be completely elastic material particles 
incapable of much deformation, and van der Waals’ view,’ 
that the properties of molecules must be compared with those of 
solids, Zsigmondy® assumes, as seems necessary, that the larger 
ultramicrons of a solid are themselves solid. The liquid proper- 
ties of gels rich in water are explained by assuming that the 
ultramicrons are surrounded by water layers and have a certain 
free path and motion. Hatschek® has examined the emulsion 
hypothesis critically and finds it untenable if the assumptions 
necessary to allow of mathematical treatment are granted. 

The assumption that jellies are solid-liquid systems is the oldest 
and is generally looked upon as the most satisfactory of the three 
general theories that have been proposed. In spite of all the 
work that has been done, there is, however, still a considerable 
difference of opinion as to the exact nature of the solid framework 
which is assumed to entrain the liquid phase and the manner in 
which this framework is formed. ‘The earliest view was that a 
distensible body was porous, and that swelling resulted from 
water penetrating the pores and being held by capillarity or by 


6 ‘Vorlesungen iiber Gastheorie,’’ Leipsig, 1896, p. 34. 

7“TDie Kontinuitaét des gasf6rmigen und Flussigen Umstanden,’’ Leipsig, 
1899, p. 34. 

8 “Chemistry of Colloids,”’ translated by Spear, 1917, p. 138. 

9° Trans. Faraday Soc., 12 (1916), 17. 


380 COLLOIDAL BEHAVIOR 


molecular attraction. In 1858, Nageli!® pointed out that porous 
bodies and gels have such widely different properties that a 
theory based on their apparent similarity is untenable. As a 
substitute theory, he suggested that distensible bodies are made 
up of small anisotropic, crystal-like molecular aggregates which 
retain their identity even when the substance goes into (colloidal) 
solution. ‘The micelles, as Nageli called them, take up water in 
such a manner that they are surrounded by a water layer the 
thickness of which is determined by the relative intensity of the 
attraction of the micelles for water and for each other. Jellies 
are thus considered to possess an interlacing or sponge structure. 
This conception was opposed by Biitschli!! and by van Bem- 
-melen,'? who suggested that the droplets of liquid were held in a 
cell-like framework comparable to a honeycomb. This idea was 
probably suggested by the cellular structure of the stems of young 
plants, which enclose a relatively high percentage of water and still 
possess considerable rigidity. Biitschli supported his view by an 
extended series of observations first on foams and emulsions and 


later on gelatin, agar, and silicic acid jellies. He found in 


certain jellies a micro structure consisting of coalescing films 
containing water. Gelatin jellies that were homogeneous were 
hardened with alcohol or chromic acid in order to make their 
structure visible, and these likewise showed the presence of 
thin films. In a silica gel the films appeared to be about 0.3 
in thickness and the pockets which held the liquid from 1 to 
1.5u in diameter. Biitschli’s view was supported by Quincke 
and by Hardy, but later investigations of Zsigmondy and his 
pupils showed that the hollows are very much finer than Biitschli’s 
observations led him to believe. By applying the laws of 
capillarity to van Bemmelen’s! results on the hydration and 
dehydration of silica gel, Zsigmondy*® estimated the diameter of 


+0 “Pflanzenphyslogischen Untersuchen,” Zurich, 1858 ; “Theorie der 
Garung,’”’ Munich, 1879; Cf. FrRanKENSTEIN: “Die Lehre von der Koha- 
sion,” Breslau, 1835. 

1 “Untersuchen tiber Strukturen,” Leipsig, 1898. 

2 Z. anorg. Chem., 18 (1898), 14. 

8 Drude’s Ann., 9 (1902), 793, 969; 10 (1903), 478, 673. 

“4 Z. physik, Chem., 33 (1900), 326. 

8 “Die Absorption,” 1910, p. 78. 

16 Z. anorg. Chem., 71 (1911), 356. 


DP Mee ts 


JELLIES AND GELATINOUS PRECIPITATES O81 


the pores to be 0.5uu, that is, 200 or 3800 times smaller than 
Biitschli observed. This was confirmed by Anderson,!7 who 
showed that the pores vary in size, some being as small as 10uu 
in diameter. Working by the same method, Bachmann!® found 
that gelatin jellies hardened by alcohol or chromic acid contained 
very much finer spaces than Biitschli supposed. Apparently, 
the structures observed by Biitschli, van Bemmelen, and Hardy 
were artifacts produced by the action of the hardening agents on 
the structure already existing. !° 

Hardy*® believes that gelatin jellies consist of two phases 
separated by a well-defined surface; one phase, a solid solution of 
‘gelatin in water, and the other, a solution of water in gelatin. 
Both phases are liquid at first but with fall of temperature one 
becomes solid. The solid-solution phase forms on the concave 
side of the surface of separation when the proportion of gelatin 
is small, and on the convex side when the proportion of gelatin 
is large. In the latter case drops of liquid are held in a solid 
gelatin-rich phase. Bancroft?! points out that such a jelly 
consists merely of a viscous medium in which liquid is dispersed 
and that it does not have a honeycomb structure in the sense 
that an emulsion has a honeycomb structure. ‘Thompson?? 
assumes that gelatin “consists of a network of solid gelatin, 
molecular or at least extremely fine, with pure water in the inter- 
stices.”’ The view entertained by Bancroft differs somewhat 
from that of either Hardy or Thompson. The former points 
out that, since water peptizes gelatin under certain conditions, 
there is no reason why gelatin or a gelatin-rich phase should not 
peptize water. Accordingly, he considers that the gelatin-rich 
phase will always contain peptized water and the water-rich 
phase will always contain peptized gelatin. The separate phases 
will, therefore, in the nature of things, never be homogeneous. 


17 Z, physik. Chem., 88 (1914), 191. 
18 Z. anorg. Chem., 100 (1917), 1. 
19 Of, Pauui: ‘‘Der Kolloidale Zustand und die Vorgange in der lebendigen 


Substanz, Braunschweig, 1902; Fiscnrer, A.: ‘‘Fixerung, Farbung und 
Bau des Protoplasms,”’ 1899, p. 312. 
a8 Toc. cit. 


21 “ Applied Colloid Chemistry,”’ 1921, p. 241. 
22 J. Soc. Leather Trades’ Chemists, 3 (1919), 209. 


382 COLLOIDAL BEHAVIOR 


Lloyd? amplifies the general view of Hardy and believes that a 
gelatin-jelly consists of two phases: a porous but continuous 
solid cellular framework, and liquid. The gelatin is assumed to 
exist in two chemical states: gelatin, per se, and gelatin in the 
form of soluble salts. Such jellies are systems of three compo- 
nents: water, gelatin, and an acid or base. On cooling a solution 
containing isoelectric gelatin and gelatin salts in equilibrium with 
free electrolytes, the insoluble isoelectric gelatin precipitates, 
not as crystals, but in a state of suspended crystallization forming 
a solid framework, which is kept extended by the osmotic pressure 
of the soluble gelatin salts in solution. In support of this 
hypothesis, isoelectric gelatin and water in the absence of so-called 
gelatin salts in solution were found to form an unstable clot 
that contracted and squeezed out liquid. It would seem, there- 
fore, that an electrolyte must be present to form a stable gelatin 
jelly in accord with the view of Jordis.24 J. Alexander?® suggests 
that what Lloyd calls “suspended crystallization” may be a mani- 
festation of the protective or crystal-inhibiting action of a portion 
of the gelatin solution. This would account for the fact that a 
jelly formed of isoelectric gelatin and water alone is apparently 
unstable, in the sense that it contracts and squeezes out some of 
the water. On account of the slight inherent tendency of gelatin 
to crystallize, it is doubtful whether the alleged increase in 
stability of a gel in the presence of a trace of electrolyte is due to 
inhibition of the crystallization of the gelatin phase. It seems 
more probable that the presence of an ion that is adsorbed may 
influence the nature and size?* of the agglomerated particles and 
so may have an effect on the stability. Apparently, the amount 
of electrolyte necessary to form a stable jelly is very slight 
indeed, since Field?’ prepared such a jelly from a very highly 
purified gelatin. | 

Lloyd’s conception of the process of gelation is criticized by 
Sheppard and Elliott?® on the ground that if the isoelectric 
gelatin forms a rigid solid framework, there is no need of postulat- 


3 Biochem. J., 14 (1920), 165. 

*4 Z. Elektrochem., 8 (1902), 677. 

* “Glue and Gelatin,” 1923, p. 71. 

6 WEISER: J. Phys. Chem., 21 (1917), 314. 
7 J. Am. Chem. Soc., 43 (1921), 667. 

8 J. Am. Chem. Soc., 44 (1922), 373. 


CL 


JELLIES AND GELATINOUS PRECIPITATES 383 


ing the existence of osmotic pressure to keep the jelly extended. 
Sheppard and Elliott?® believe that any structure in a gelatin 
jelly is not inherent in the gelatin, but that the structure ele- 
ments are resultants of physico-chemical changes of environment, 
not native to gelatin. Following up Meunier’s®® view, they 
postulate a supermolecular rather than a submicroscopic struc- 
ture for gelatin. According to this view, pluri-molecular units, 
the smallest of which are the micelles, are built up by the “ orien- 
tation of definite atomic groups entirely in the sense of the theory 
of molecular orientation due tostructure, proposed for surface and 
interfacial tension phenomena by W. B. Hardy,*! W. Harkins,*? 
and J. Langmuir.’’*? 

The earliest investigations with the ultramicroscope on gelatin 
and semi-liquid hydrosols led Zsigmondy to conclude with 
Nageli that the structure is granular or flocculent, a view that is 
supported by the fact that jellies can be formed from freely mov- 
ing ultramicrons.*4 Later, Zsigmondy and Bachmann* pointed 
out that, in addition to the apparently grainy structure met with 
in diluted gels of gelatin, agar, and silica, there is also a fibrillar 
structure. These fibrils or threads are sharply defined in soap 
jellies studied by Bachmann and later by McBain and his 
co-workers*® and in barium malonate jellies studied by Flade.*’ 
The latter noted that the fibrils were of a crystalline character and 
suggested that jellies in general probably consisted of a network 
of crystalline threads. Stubel** and Howell*® concur in this view, 
the latter introducing the term ‘crystalline gel.’’ Gortner?° 
prepared a jelly of dibenzoyl-l-cystine, which was found to con- 


20 Cf. SHEPPARD: Nature, 107 (1921), 73. 

30 Chimie & industrie, 6 (1920), 220. 

31 Proc. Roy. Soc., 81 A (1912), 610. 

32 J, Am. Chem. Soc., 39 (1917), 354, 541. 

33 [hid., 38 (1916), 2221. 

34 BacHMANN: Z. anorg. Chem., 25 (1911), 125. 

35 Kolloid-Z., 11 (1912), 150. 

36 Tarnc and McBatn: J. Chem. Soc., 117 (1920), 1506; Drake, McBain 
and Satomon: Proc. Roy. Soc., London, 98 A (1921), 395. 

87 Z. anorg. Chem., 82 (1913), 178. 

8 Pfliiger’s Archiv., 156 (1914), 361. 

39 Am. J. Phys., 40 (1916), 526. 

40 7, Am. Chem. Soc., 48 (1921), 2199. 


384 COLLOIDAL BEHAVIOR 


sist of minute crystalline needle-like fibrils. Bradford4! cham- 
pions the theory that the reversible sol-gel transformation is merely 
anextreme case of crystallization. Ultramicroscopic examination 
of a gelatin jelly reveals the presence of spherites, which Bradford 
believes are made up of crystalline particles. Moeller42likewise 
believes that gelatinization is a kind of crystallization in which 
there is formed a lattice of crystal threads which entrains the 
liquid; and von Weimarn* concludes from his investigations that a 
jelly is a sponge composed of highly disperse, crystalline granules 
soaked in dispersive medium. 

While Bradford, Moeller, and von Weimarn may have sufficient 
evidence to convince them that all jellies are made up ultimately 
of crystals, it is difficult to accept the view that there is no such 
thing as an amorphous precipitate of the flocculent, gelatinous, or 
jelly-like type. The theory that jelly formation is merely a 
process of crystallization seems to be contradicted by the work 
of Bogue, McBain, and Barratt, although all of the latter 
are strong supporters of a filamentous structure. Bogue‘ 
believes that the elastic jellies such as gelatin are made up of 
streptococcal threads of molecules: 


The sol consists of slightly hydrated or swollen molecules united into 
short chains. When the temperature falls, the threads increase in 
length and number and their power of water absorption increases, 
resulting in an increase in viscosity. A solid jelly results when the 
relative volume occupied by the swollen molecular threads has become 
so great that freedom of motion is lost and the adjacent heavily swollen 
aggregates cohere. The rigidity is dependent on the relative amount 
of free solvent in the interstices of the aggregates and on the amount 
of solvent that has been taken up by the gelatin in a hydrated or imbibed 
condition. The resiliency or elasticity is dependent upon the length 
and number of the catenary threads. Solution is the reverse of gela- 
tion. Swelling is determined by osmotic forces and the Donnan 
equilibrium. 

While in certain cases the colloidal particles—the molecular 
aggregates, or micelles—may possess the thread-like characteristic 
essential for forming an entangling mesh in which each particle 


‘t Biochem. J., 12 (1918), 351; 14 (1920), 474. 

“2 Kolloid-Z., 23 (1918), 11. 

*° J. Russ. Phys. Chem. Soc., 47 (1915), 2163. 

** Chem. Met. Eng., 23 (1920), 61; J. Am. Chem. Soc., 44 (1922), 1343. 


JELLIES AND GELATINOUS PRECIPITATES 385 


is discrete, in other cases it is probable that the micelles actually 
become stuck together or oriented into loose aggregates which 
may take the form of chance granules, threads, or chains. Such 
a linking together of the particles to form an enmeshing network 
seems essential in some of the extremely dilute inorganic jellies 
which will be referred to later on. Laing and McBain consider 
the gelatinization of soap to result from the linking up of colloidal 
particles to form a filamentous structure. ‘‘The colloidal parti- 
cles in soap and gel are the same; but, whereas in the former they 
are independent, in a fully formed gel they become linked up 
probably to form a filamentous structure.”? The formation of 
the soap curd is looked upon as a phenomenon analogous to 
crystallization, which is distinct from the process of jelly forma- 
tion. The conception of micellar orientation in the process of 
gelation is supported by a number of observations mentioned by 
Laing and McBain among which are the following: The identity 
in sol and gel of the electrical conductivity,*> and the lowering of 
the vapor pressure; the intensifying of the molecular movement 
by heat, which overcomes the forces holding the particles and 
causes melting of the gel; the transformation of a jelly (nitro- 
cotton) into a sol by mechanical stirring, which breaks down the 
orienting bonds betwen the particles;** the absence of Brownian 
movement in soap or gelatin jellies;4” the dependence of the 
apparent viscosity of sols on their previous treatment and history, 
which influences, the degree of orientation of their particles;*® 
and the frequent occurrence of supersaturation and hysteresis 
with regard to gelation. ‘To these should be added the observa- 
tion of Walpole*® that the refractive index of a gelatin water 
system is a linear function of the concentration, and, when plotted 
against the temperature, no break occurs at the point of gelation; 
and the findings of Bogue®® that the viscosity-plasticity change in 
the sol-gel transformation is gradual and regular. 


45 Cf, ARRHENIUS: Oefvers. Stockholm Akad., 6 (1887), 121. 
46 Cf. ALEXANDER: ‘‘Glue and Gelatin,’’ 1923, p. 75. 

47 BACHMANN: Z. anorg. Chem., 73 (1912), 125. 

4 Cf. HatscHEK: Kolloid-Z., 13 (1913), 881. 

4 Kolloid-Z., 13 (1913), 241. 

50 J, Am. Chem. Soc., 44 (1922), 1318. 


386 COLLOIDAL BEHAVIOR 


Barratt*' observed a distinct fibrillary structure in fibrin jellies 
but found that they did not possess the physical character of 
crystals. Like Laing and McBain, he is of the opinion that the 
fibrils form a network structure. When a jelly was first formed 
by gelatinization of a fibrinogen sol, no fibrils could be detected, 
but later they became visible in the ultramicroscope. This 
led to the conclusion that the jelly was made up of fibrils that 
were submicroscopic at first and later ultramicroscopic. This 
growth of particles in jellies has been observed frequently and in 
some cases is unquestionably due to growth of crystals, notably 
with barium malonate and some of the arsenate jellies®? and with 
the dyes, benzopurpurine and chrysophenene ;°° but in other cases | 
it is the result of the agglomeration of amorphous particles. In 
accord with this view, Scherrer‘ showed that certain rigid jellies, 
like silicic acid and stannic acid, showed well-defined crystalline 
interference figures as well as the characteristics of amorphous 
bodies, whereas gelatin jellies showed no signs of a crystalline 
structure. Harrison®> obtained spherical coagulation forms of 
starch which resembled Bradford’s spherites; but he does not 
regard them as crystalline. As already stated, Zsigmondy and 
Bachmann observed ultramicroscopically the formation of jellies 
of gelatin, agar, and silica by agglomeration into flaky groups of 
freely movable ultramicrons of unknown structure. It is thus 
implied that all jellies are not necessarily filamentous in structure. 
This is supported by recent ultramicroscopic observations carried 
out by Harrison®® on gelatin and cellulose jellies, which were 
found to consist of minute portions joined together in a somewhat 
irregular manner. Alexander®’ believes that the polar nature 
of molecules may tend to produce some kind of orientation and 
that some chain-like structures may be formed, but that the 
formation of chains or threads is not an essential of jelly formation. 


5t Biochem. J., 14 (1920), 189. 

*? Digsz: Kolloid-Z., 14 (1914), 139. 

°’ HARRISON: ‘‘The Physics and Chemistry of Colloids and Their Bearing 
on Industrial Questions,’ Report of a General Discussion held jointly by 
the Faraday Society and Physical Societies of London, Oct. 25, 1920, p 57. 

4 Nach. Ges. Wiss., Gottingen, 1918. 

°° J. Soc. Dyers, Colorists, 82 (1916), 32. 

56 Loc. cit., Ref. No: 53, p. 57. 

7 “Glue and Gelatin,” 1923, p. 84. 


JELLIES AND GELATINOUS PRECIPITATES 387 


He admits, however, that polar groupings in chains probably 
takes place to a considerable extent in dilute solutions of gelatin. 

It would be highly interesting indeed if jellies of widely differ- 
ent substances were all essentially identical in structure. Sucha 
condition seems altogether unlikely, but investigators have 
apparently sought to establish such an identity. Studies on 
specific jellies have led some to conclude that all jellies are made 
up of a framework of amorphous threads, others that they are 
composed of crystalline threads, and still others who fail to find 
any threadsor filaments at all but observe an irregular grouping of 
particles. Doubtless all are right in specific cases. Indeed, 
it is not unlikely that there are various arrangements of molecular 
aggregates in different jellies and, perhaps, in the same jelly. In 
a heterogeneous mixture of complex groups, such as are found in 
gelatin sol or jelly, it is probable that the process of gelation and 
the jelly structure are more complex than in the inorganic jellies 
or in soap jellies. The orientation of the particles may result in 
fibrils in certain cases and in more or less irregular arrangements 
in others. In certain cases the fibrils may consist of definite crys- 
tals, while in others the crystalline characteristics may be entirely 
lacking. In all cases it seems probable that the particles are 
highly hydrous as a result of adsorption or absorption and that 
they are linked together, forming an irregular mesh or network in 
the interstices of which liquid is entrained. 

Since the only essential difference between a jelly and a gelati- 
nous precipitate appears to be that in the latter case contraction 
has taken place with the excretion of liquid, it is generally con- 
ceded that the two types of gels are similar in structure. While 
the usual gelatinous precipitates, such as hydrous chromic 
oxide and hydrous ferric oxide, are pretty generally considered 
to be amorphous, von Weimarn believes them to be made up of 
myriads of tiny crystals. It is apparently possible to have 
gelatinous crystals. Thus, Harrison*®* speaks of aqueous solu- 
tions of benzopurpurine and chrysophenene setting to jellies 
containing gelatinous crystals, some of them so fine that they 
can pass unbroken through a filter paper. Similarly, cholic acid 
gives a blue precipitate with iodine, which forms in clusters of 
needle crystals which are rigid. Under other conditions needle- 

88 Loc. cit., Ref. No. 53, p. 58. 


388 COLLOIDAL BEHAVIOR 


shaped crystals are formed which are gelatinous and can be bent 
in all shapes by moving the cover glass on the microscope slide. 
Some of these so-called gelatinous crystals show remarkable 
vibrations due to the impact of the molecules. Harrison’s 
observations seem to throw some light on the problem of what 
constitutes a gelatinous crystal or aggregate, and hence on the 
related problem of what is a gelatinous precipitate. 

Le Chatelier®® succeeded in polishing metal with colloidal 
silicic acid and hence concluded that the gelatinous precipitate 
consists of anhydrous silica and water. Bancroft®® considers 
this evidence inconclusive, since anhydrous silica may have 
been formed as a result of pressure during polishing, and suggests 
that a better method of attack is to consider whether grains of 
sand mixed with water will give a gelatinous precipitate. Since 
this does not happen, Bancroft concludes: 

We must, therefore, assume one of two things. Either the sand 
grains are held together extraordinarily firmly by water when they are 
very fine, or some other factor comes in. The first explanation can- 
not be the right one, because, if it were, one ought then to be able to 
get a gelatinous precipitate of any colloid at ordinary temperatures 
without much difficulty, which is not the case. We never get gelatinous 
gold and, while we can get gelatinous calcium carbonate, we have to 
do it in a very special way. Consequently, Le Chatelier’s hypothesis 
cannot be accepted without modification. 

Bancroft cites evidence to support the hypothesis that a gelat- 
inous precipitate is a two-phase liquid-liquid system, the phys- 
ical properties of which are determined by the viscous character 
of one of the phases. This view seems unsatisfactory and in 
certain cases at least is untenable. It is suggested further that 
solid particles and water may behave like a gelatinous precipitate 
when the solid particles are sufficiently fine and provided 
they adsorb water sufficiently strongly. As previously noted, 
Zsigmondy®! explains the liquid character of gels rich in water by 
assuming that the ultramicrons are surrounded by water layers 
and have a certain free path and motion. Bancroft objects to 
this view on the ground that Zsigmondy does not show why it 
should be so. MHarrison’s observations on gelatinous crystals 


69 “Ta Silice et Les Silicates,”’ 1914, p. 76. 
60 ** Applied Colloid Chemistry,’’ 1921, p. 236. 
61 ‘Chemistry of Colloids,” translated by Spear, 1917, p. 138. 


JELLIES AND GELATINOUS PRECIPITATES 389 


bear on this point. Gelatinous crystals are apparently extremely 
fine needle-shaped masses, so thin that they lack rigidity and 
so flexible that they can be bent and twisted into various shapes 
and may move under the bombardment of water molecules 
like the spiral bacteria present on the teeth. A cluster or net- 
work of such needle-shaped flexible crystals which adsorb water 
strongly would form a viscous or plastic mass which is usually 
known as a gelatinous precipitate. If the crystals are compact 
and rigid rather than thin and flexible, they would not form a 
gelatinous precipitate unless they united into threads or strings 
that would possess the flexibility and elasticity which characterize 
a thin needle crystal. Obviously, the particles need not be crys- 
talline and, as a rule, they probably are not. A gelatinous pre- 
cipitate is apparently a network composed of extremely finely 
divided particles which have coalesced to form flexible filaments 
or chains, and which adsorb water strongly, and so are highly 
hydrous. Where the particles do not adsorb water particularly 
strongly and where the tendency to coalesce into filaments or 
threads is not great, a high concentration of the finely divided 
particles is necessary, as in the case of calcium carbonate and 
barium sulfate. It is probable that neither tendency is very 
marked in the case of gold, which accounts for the fact that no 
one has prepared a gold jelly. However, the writer is not aware 
that anyone has attempted to precipitate a fairly large amount 
of gold in a small volume, as von Weimarn does with barium 
sulfate. While a gelatinous precipitate of gold has not yet 
been prepared, this might be a fairly simple process if some liquid 
other than water were employed. Béorjeson,*? working in Sved- 
berg’s laboratory, has prepared a cadmium jelly by allowing 
a very dilute sol of cadmium in alcohol to stand for some time in 
a glass bottle. In this case the particles were only 5yu in radius 
and the concentration but 0.2 to 0.5 per cent. 


FORMATION OF GELS 


If we start out with the assumption that a jelly consists of 
myriads of hydrous particles that have become enmeshed into a 
network that entrains liquids, it follows that any substance 


62 Toc. cit., Ref. No. 53, p. 55. 


390 COLLOIDAL BEHAVIOR 


should form a jelly, provided a suitable amount of a highly 
dispersed substance is precipitated and provided the particles 
adsorb the dispersing medium very strongly. The amount of 
the dispersed phase that must be present to form a firm jelly 
by a precipitation method will depend on the size and arrange- 
ment of the particles and the extent to which they adsorb the 
dispersing liquid. ‘The methods of procedure which have been 
employed will be considered separately. 

Formation of Jelly by Cooling Sol.—Certain substances, such 
as gelatin and agar-agar, swell in water at ordinary temperatures 
but are not peptized, forming a sol until the temperature is raised. 
On cooling such a sol a jelly is formed, provided the concentra- 
tion is suitable. Thus, a sol containing 1 per cent of pure gelatin 
does not gel until around 10° and gelation does not take place at 
any concentration above +35°. Bachmann®* observed that 
pure, warm solutions of gelatin are almost homogeneous but 
that, on cooling, a new phase appears, as evidenced by a hetero- 
geneity which is amicroscopic or submicroscopic, depending on 
the concentration. This process is similar in certain respects 
to crystallization but differs from it in that microns, submicrons, 
ultramicrons, and amicrons are formed, according to the concen- 
tration. The appearance of visible particles is not dependent 
on the formation of a jelly, as these may be seen before the 
jelly sets, and in dilute solutions that do not set. When a jelly 
results on cooling a sol, the process apparently consists in the 
formation of highly hydrous molecular aggregates which are 
linked together to form a more or less rigid network. Bogue 
believes that the aggregates not only grow but become more 
hydrous on cooling. This might be expected in view of the fact 
that, in general, adsorption increases rapidly with falling tempera- 
ture. The sol-gel transformation in a given system does not 
occur at a definite transition point, but the transition is continu- 
ous and reversible over a somewhat indefinite period. 

Formation of Concentrated Jellies—Many difficultly soluble 
salts which ordinarily precipitate in relatively large crystals can 
be thrown out in the form of a jelly or gelatinous precipitate from 
very concentrated solutions. ‘This phenomenon was observed by 


63 Z. anorg. Chem., T3 (1911), 125. 
64 BoauEe: J. Am. Chem. Soc., 44 (1922), 1313. 


JELLIES AND GELATINOUS PRECIPITATES 391 


Harting,®* Buchner,*® Biederman,*”’ Neuberg,*®* and particularly 
by von Weimarn.®® ‘The latter’? made a systematic study of the 
form in which substances precipitate from solution. He calls 
attention to the fact that precipitation depends on a number 
of very different factors; on the solubility of the substance; 
on the latent heat of precipitation; on the concentration at which 
the precipitation takes place; on the normal pressure at the sur- 
face of the solvent; and on the molecular weights of the solvent 
and the solute. He points out the impossibility of taking all 
of these factors into account and simplifies the problem by 
considering first but two of the factors, the solubility of the 
precipitating substance and the concentration at which precipi- 
tation begins. The effect of viscosity is discussed briefly in a 
later work.’ The process of condensation (precipitation) 
is divided into two parts: the first stage, in which the molecules 
condense to invisible or ultramicroscopic crystals; and the second, 
which is concerned with the growth of the particles as a result of 
diffusion. The velocity at the important first moment of the 
first stage of the precipitation is formulated thus: 


_ , Condensation pressure ,Q—L_ ,P _ 
Me Condensation resistance _ is ji; ey Oe =U 








where W is the initial rate of precipitation, Q the total concentra- 
tion of the substance which is to precipitate, L the solubility of 
coarse crystals of the substance, Q — L = P the amount of super- 


; od 
saturation. The ratio aes U is the percentage supersaturation 


at the moment precipitation begins. 
The velocity of the second stage is given by the Nernst-Noyes 


equation: 


D 
V= ee ea 


6 ‘“Recherches de morphologie synthétique sur la production artificielle 
de quelques formations calcaries organiques,’”’ Amsterdam, 1872. 

6 Chem. Ztg., 17 (1893), 878. 

87 Z. allgem. Physiol., 1 (1902), 154. 

88 Sitzb. Akad. Wiss., Berlin, 1907, 820. 

69 “Zur Lehre von den Zusténden der Materie,”’ 1914. 

70 Von WeErmARN: ‘‘Grundziige der Dispersoidchemie,’”’ (1911) p. 30. 

71 “Zur Lehre von den Zusténden der Materie,” (1914) p. 21; Kolloidchem. 
Bethefte, 4 (1912), 101. 


392 COLLOIDAL BEHAVIOR 


where D is the diffusion coefficient, S the thickness of the adherent 
film, O the surface, C the concentration of the surrounding solution, 
and I the solubility of the dispersed phase for a given degree of 
dispersity. C—Il may be termed the absolute supersaturation. 

From these general formulations, von Weimarn arrives at the 


gabe. Sy & 
conclusion that jellies are obtained only when the ratio 7 that 


is, the percentage supersaturation U, can be made enormous. 
It is pointed out that the nature of a precipitate is quite different, 
depending on whether a given value of U is obtained by a large 
Porbyasmall Zl. Ifa large U is obtained by a high value of P, 
a large amount of disperse phase is produced and a gel forms, 
while if P is small and LZ very small, a relatively small amount of 
disperse phase is produced and a sol is formed. Von Weimarn 
has demonstrated the accuracy of his deductions in a large 
number of cases, using reacting solutions of high concentrations; 
and it is apparently true that any salt can be obtained in a gela- 
tinous form if the concentration of the reacting solutions, and so 
the velocity of precipitation, is sufficiently high. Thus von | 
Weimarn’”’ prepared jellies of substances like BaSOu, which usually 
precipitates in the form of crystals, by mixing very concentrated 
(8 to 7 N) solutions of manganese sulfate and barium thiocy- 
anate. This is not the condition under which jellies are usually 
obtained, and their existence is temporary. By mixing very 
high concentrations of materials which react to form an insoluble 
precipitate, a very large number of relatively small particles is 
formed because of the high degree of supersaturation.’* Each 
of these minute particles adsorbs a little water and, as they are 
very close together, a semi-solid mass results, which entrains all 
the liquid phase, thus forming what has been termed a jelly. 
These so-called jellies break down on standing, on account of 
growth of the particles and the consequent liberation of adsorbed 
water. Precipitates in which the ratio of mols of water to 
mols of salt is, say, 20:1 or 25:1 should not be considered as 
jellies in the same sense as precipitates in which this ratio is two 
or three hundred times as great. Very finely divided sand or 
fuller’s earth may be matted in the bottom of a test tube and this 


72 “Zur Lehre von den Zustainden der Materie,” (1914), p. 21. 
73 Bancrort: J. Phys. Chem., 24 (1920), 100. 





JELLIES AND GELATINOUS PRECIPITATES 393 


solid will take up a great deal of water before a supernatant water 
layer is observed; but such a preparation should not be called 
a jelly. It seems that von Weimarn’s barium sulfate jelly may 
be similar, except that the particles are much smaller, and so a 
given amount will take up more water. On the other hand, with 
true jellies, where the amount of enclosed water may be relatively 
enormous, time must be allowed for the formation of a definite 
structure. As a matter of fact, von Weimarn™ recognized a 
difference between a BaSO, jelly prepared by his method and a 
jelly formed by uniform gelatinization of a liquid throughout 
its mass, as in the case of gelatin jelly. The former he terms a 
“coarsely cellular gel” and the latter a “reticulated gel.” 
Formation of Jellies by Precipitation from Sol.—Since finely 
divided particles which adsorb water strongly are of primary 
importance for the formation of a hydrous jelly, it would seem 
that the most promising method of preparing dilute jellies would 
be to precipitate hydrous substances from colloidal solution. 
The von Weimarn theory would tell us, of course, that this pre- 
cipitation would have to take place at a suitable rate under 
conditions that are not conducive to growth of the individual 
particles, but it does not enable us to predict the optimum rate of 
coagulation, the effect of salts on jelly formation, or the condi- 
tions which determine the formation of a jelly rather than a 
gelatinous precipitate. As a result of recent investigations on 
the formation of typical dilute inorganic jellies, the writer has 
outlined the general conditions of jelly formation and the effect 
on the process of various factors other than the percentage super- 
saturation ‘‘at the important first moment of the first stage of 
condensation”? from molecules to invisible crystals. Jellies 
would be expected to form from colloidal solution if a suitable 
amount is precipitated at a suitable rate without agitation in the 
absence of a medium that exerts an appreciable solvent or pep- 
tizing action. If the concentration of the colloid is too low, no 
jelly, or only a very soft jelly, can result. If the velocity of 
precipitation is too great, contraction is likely to occur, with the 
formation of a gelatinous precipitate instead of a jelly. The 
effect of the presence of salts on jelly formation is, therefore, 
determined, in large measure, by the precipitating and stabilizing 
74 J. Russ. Phys. Chem. Soc., 47 (1915), 2163. 


394 COLLOIDAL BEHAVIOR 


action of the ions in so far as these affect the rate of precipitation. 
In general, a slow rate of precipitation is to be preferred if there 
is little or no tendency of the particles to grow as a result of the 
solvent action of the electrolyte. ‘The favorable concentration 
for different electrolytes is in the immediate region of their 
precipitation concentration. A little below this value no precipi- 
tation, or only a slight precipitation, takes place, while above this 
value coagulation is usually so rapid that a gelatinous precipitate 
is formed instead of a jelly. The reasonisthat time is not allowed 
for the uniform mixing of the colloid with coagulant, and the 
slow uniform precipitation necessary for the building of a uniform 
jelly structure is replaced by rapid uneven coagulation and the 
consequent contraction which distinguishes a gelatinous pre- 
cipitate from a Jelly. 

The accuracy of these deductions has been demonstrated 
repeatedly in the writer’s laboratory using sols of the hydrous 
oxides of chromium” (both positive and negative), tin,’® copper,”” 
aluminum, and the arsenates of iron and aluminum.’* In many 
cases these jellies may be obtained in relatively low concentra- 
tion. A notable example is the case of chromic oxide, which 
formed a firm jelly containing but 0.18 per cent CreO; and a soft 
jelly containing 0.09 per cent CreO3. Firm jellies of such low 
concentrations are comparatively rare, though Déehle and 
Rassow”® obtained jellies of the mercury salt of an organic sulfo 
acid in concentrations of 0.72 per cent, while Foerster found that 
camphorylphenylthiosemicarbazide formed stiff jellies in con- 
centrations of 0.383 per cent and trembling jellies in concentra- 
tions of 0.25 per cent.8° Attention has already been called to 
Boérjeson’s cadmium jelly and to Gortner’s dibenzoyl-l-cystine 
jelly, both of which could be prepared in concentrations as low 
as 0.20 per cent. The formation of such dilute jellies results 
only when the particles are very hydrous and when the conditions 
of precipitation allow time for the building up of an enmeshing 


7% WauisER: J. Phys. Chem., 26 (1922), 419, 424. 
7% Ibid., 26 (1922), 689. 

7 [bid., 27 (1923), 685. 

78 Ibid., 28 (1924), 1. 

79 Kolloid-Z., 12 (1913), 71. 

80 HatscHEeK: Jbid., 11 (1912), 158. 





JELLIES AND GELATINOUS PRECIPITATES 095 


network. In case the particles are but slightly hydrous and show 
but little tendency to link together into threads, extremely high 
concentrations must be present, as von Weimarn found. Unfor- 
tunately, there has been no systematic work done on the forma- 
tion of jellies from non-aqueous sols. 

Formation of Jellies by Dialysis of Sol.—On dialysis of a 
colloidal solution of ferric arsenate peptized by ferric chloride, 
Grimaux*! obtained a firm, transparent jelly. This observation 
has been confirmed and extended by Holmes and his pupils.*? 
Similar observations have been made in the writer’s laboratory with 
hydrous chromic oxide, and the method is probably a general one. 
From the point of view outlined in the foregoing section, the 
formation of jellies by dialysis of a colloidal hydrous substance 
is readily understood. Dialysis merely removes the stabilizing 
ion slowly and uniformly below the critical value necessary for 
peptization, and precipitation results just as if the adsorption of 
the stabilizing ion were compensated for or neutralized by the 
addition of an electrolyte having a suitable precipitating ion. 
The accuracy of these deductions has been demonstrated conclu- 
sively in a series of investigations on the arsenates of iron and 
aluminum. *? 

Formation of Dilute Jellies by Metathesis.—The von Weimarn 
theory tells us that mixing dilute solutions which interact at 


once will not give a jelly, since the percentage supersaturation 
ys : 

fa U is too small because of the small value of P. Asa 
matter of fact, however, jellies have been obtained under certain 
conditions by mixing very dilute solutions, in which ZL is suffi- 
ciently large that precipitation is slow and quantitative precipi- 


P ; 
tation impossible, so that pon U is small. Such cases are 


apparently not covered by the von Weimarn theory. It is 
quite possible to obtain a gelatinous precipitate by mixing dilute 
solutions of two salts which precipitate immediately (P small, 


81 Compt. rend., 98 (1884), 1540. 

82 Houtmes and RinpFrusz: J. Am. Chem. Soc., 38 (1916), 1970; HotmeEs 
and Arnotp: Jbid., 40 (1918), 1014; Houmzs and Fatt: Jbid., 41 (1919), 
763. 

83 WerisER: J. Phys. Chem., 28 (1924), 26. 


396 COLLOIDAL BEHAVIOR 


but L very small), but a jelly will not form under these conditions. 
The reason is evident when we consider the impossibility of 
obtaining the instantaneous mixing of the solutions which is 
essential for uniform precipitation throughout the solution. One 
part is precipitated before another is mixed with the precipitant, 
and the uniformity which is characteristic of a jelly is lost. 
Moreover, the mixing itself will tend to destroy the jelly structure. 
The results are, therefore, not unlike those obtained when a 
colloid capable of forming a jelly by slow precipitation is coagu- 
lated too rapidly by the addition of excess electrolyte. To obtain 
a jelly from a colloidal solution, it is necessary to add such an 
amount of electrolyte that thorough mixing is possible before 
appreciable coagulation takes place. From these considerations 
it follows that precipitation of a hydrous substance as a result 
of double decomposition might form a jelly instead of a gelatinous 
precipitate, in case the thorough mixing of the solutions could 
be effected before precipitation began, and in case the precipita- 
tion once started, proceeded at a suitable rate. Such conditions 
do not obtain as a rule, but they are quite possible theoretically. 
Thus, the precipitation may be the result of a stepwise process, 
one step of which proceeds at a suitably slow rate. It is further 
possible to have a reaction which proceeds very slowly at low 
temperatures but with marked velocity at higher temperatures. 
This would not only allow of mixing without precipitation but 
would enable one to control the subsequent rate of reaction by a 
suitable regulation of the temperature. 

Such a favorable combination of circumstances apparently 
obtains when a manganese salt of a strong acid and KH2AsO, are 
mixed. ‘The latter salt ionizes thus: KH,.AsO.@Kt + H;As0,; 
but, on account of the solubility of Mn(H2AsO,), no Mn** ions 
are removed from solution by interaction with H,AsOs-. ‘The 
latter ion undergoes secondary ionization to a slight degree, 
however, as follows: H,AsO,@H+ + HAsO,; and insoluble 
MnHAs0, is formed in accord wth the reaction: Mn++ + HAsO, 
= MnHAsO,.*4 

Since the precipitation of MnHAsO, is accompanied by the 
formation of an equivalent amount of free hydrogen ion in solu- 
tion, an equilibrium is set up which prevents the complete pre- 

84 Drmsz.: Kolloid-Z., 14 (1914), 189. 








JELLIES AND GELATINOUS PRECIPITATES 397 


cipitation of the manganese. The amount of MnHAsO, formed 
and the rate of formation by the above process, however, are 
apparently influenced to a marked degree by the temperature, so 
that it should be possible to obtain good jellies by mixing dilute 
solutions of the necessary salts in the cold and allowing the mixture 
to stand at room temperature or warming to asuitable temperature. 
This was demonstrated with the arsenates of manganese, cobalt, 
iron, cadmium, and zine.*® When the precipitated particles 
are very highly hydrous, and when the tendency to crystallize 
is slight, very dilute jellies may be prepared by this method. 
Thus, a firm jelly was formed with 0.5 per cent, and a soft jelly 
with but 0.25 per cent, MnHAsO,. On the other hand, where 
there is a marked tendency to crystallize, a permanent jelly cannot 
be obtained, as in the case of cadmium arsenate. If the condi- 
tions were altered so as to bring about very rapid precipitation, a 
gelatinous precipitate instead of ajelly wasthrowndown. Flade*® 
has examined the manganese arsenate jelly microscopically and 
has found it to consist of fibrils or filaments like the barium malo- 
nate jellies. Here again the time factor is important for the for- 
mation of an enmeshing network of hydrous filaments. 
Swelling.—Practically all substances which form the so-called 
elastic gels show the capacity of swelling in a suitable liquid. 
Thus, dry gelatin, fibrin, and starch will swell in water at ordinary 
temperatures, forming jellies that are peptized at higher tempera- 
tures, forming sols. Similarly, albumin swells in water but not in 
alcohol, benzol, ether, or turpentine. Vulcanized india rubber 
swells in various organic solvents, such as benzol, toluol, and 
xylol, but not in water; and soaps swell in water and in many 
organic solvents. Numerous theories’? have been advanced 
to explain the phenomenon, but there is as yet no explanation 
that suffices to account for the fact that certain substances swell 
in only a limited number of liquids. The swelling of gelatin 
has been studied most extensively and has been found to depend 
on a number of factors, among which may be mentioned the 


8 Waisrer: J. Phys. Chem., 28 (1924), 26. 

86 Note in Kolloid-Z., 14 (1914), 141. 

87 These theories have been summarized and their limitations pointed 
out in a paper by BarText and Sims: J. Am. Chem. Soc., 44 (1922), 289. 


398 COLLOIDAL BEHAVIOR 


hydrogen ion concentration,*® the addition of neutral salts,*® 
the temperature, and the structure.°® The importance of the 
hydrogen ion concentration on the swelling phenomenon was 
suggested by Ostwald and has been emphasized particularly by 
Procter and by Loeb. Fischer believes that the effect of acids is 
not the same at the same hydrogen ion concentration on account 
of the influence of the anion of the acid, which varies as indicated 
by the well-known lyotrope or Hofmeister series. Loeb found that 
only the anions of neutral salts were taken up by gelatin on the 
acid side of the isoelectric point (pH 4.7) and only cations on 
the alkaline side, whereas both ions of a neutral salt would be 
expected to have an effect in accord with the Hofmeister series. 
As amatter of fact, Loeb was working with very low concentra- 
tions of salts, and so detected no effect of cations other than 
hydrogen on the acid side and of anions other than hydroxyl on 
the basic side. At higher concentration of neutral salts the 
specific effect of ions other than hydrogen and hydroxyl would 
doubtless appear. ‘This inference is supported by work carried 
out in the writer’s laboratory on the adsorption of anions by 
hydrous chromic oxide on the alkaline side of the isoelectric point. 
If the concentration of the anion under consideration is very 
large relatively to that of hydroxyl, the effect of the latter is 
negligible, whereas if the hydroxyl ion concentration is appreci- 
able, the adsorption of the other ion is cut down enormously and 
may be completely nullified. 

As previously noted, Procter and Loeb champion the theory 
that gelatin forms readily soluble and highly ionized salts, such 
as gelatin chloride and sodium gelatinate, and that the osmotic 
pressure of these salts and the Donnan equilibrium determine 
the swelling of a gelatin jelly. While this theory may explain 
adequately the swelling phenomena of gelatin, it is apparently 
inapplicable to such cases as the swelling of rubber in benzol or 


88 CuraRi: Biochem. Z., 38 (1911), 167; Procter: J. Chem. Soc., 105 
(1914), 313; Lozs: J. Gen. Phystol., 1 (1918), 41. 

89 HOFMEISTER: Archiv. experim. Pathol., 27 (1890), 395; 28 (1891), 210; 
Pauui: Pfltiger’s Archiv., 67 (1897), 219; 71 (1898), 333; Sprro: Bettrage 
Chem. Physiol., 5 (1904), 276; Ostwaup: Pfltiger’s Archiv., 108 (1905), 563; 
FiscHEr: ‘‘Edema,’’ New York, 1910. 

90 PRocTER and Burton: J. Soc. Chem. Ind., 35 (1916), 404; Arisz: 
Kolloidchem. Bethefte, T (1915), 49. 








JELLIES AND GELATINOUS PRECIPITATES 399 


xylol, where the existence of a Donnan equilibrium is precluded 
by the absence of dissociation.®! 

The previous history of a piece of dry gelatin seems to have a 
very marked effect on its capacity to absorb water and swell. 
Thus, Arisz prepared a 20 per cent jelly which was allowed to 
swell to a 10 per cent jelly. The 10 per cent jelly prepared in 
this way would not take up as much additional water in a reason- 
able time as a jelly made up at 10 per cent directly. Bancroft 
reports that Cartledge prepared 8, 16, 24, and 32 per cent gelatin 
jellies which were dried at room temperature to a concentration of 
about 96 per cent. When placed in water, each swelled rapidly 
to the original concentration and then took up water but slowly. 
B. Humiston made similar observations under the writer’s direc- 
tion at the Catholic University branch of the American University 
Experiment Station during the war. ‘These observations indicate 
that the structure of gelatin may have a marked influence on 
its capacity to swell, a possibility which will have to be taken into 
account in any theory of the swelling process. 

As noted above, the dehydration and swelling of a gelatin jelly 
is reversible over a considerable range. This is not the case with 
a silicic acid jelly. Van Bemmelen®? showed that a silica gel 
containing a great deal of water shrinks very much when the 
water is removed; and, while it will take up some water again, 
the volume change is not reversible. If the drying is carried 
sufficiently far, pores are developed which are filled with air, 
and these pores can then be filled with a liquid other than water; 
but there is no appreciable swelling. When gelatin is dried, such 
pores are not developed and a dry gel of natural gelatin will not 
adsorb benzene. 

Although the porous mass formed by drying a non-elastic gel 
will not swell in organic liquids, Graham found that such liquids 
will replace the water in a jelly. Thus, a silica jelly containing 
11 per cent SiOz was suspended repeatedly in alcohol and an 
alcogel was formed having approximately the same volume as the 
original jelly. Ina similar way, the water was replaced by inor- 
ganic and organic acids. Van Bemmelen substituted acetone for 


1 Fiscuer, M. H., in ‘‘Glue and Gelatin,” by ALEXANDER, J., p. 93. 
92 “Die Absorption,” 1910. 


400 COLLOIDAL BEHAVIOR 


the water, and Bachmann substituted benzol. Neuhausen 
and Patrick®4 found that the replacement of water was not so 
complete as Graham reported on repeated immersions of a silica 
jelly in anhydrous alcohol or benzol. Elastic jellies show a 
similar behavior. Thus, Biitschli®® found that it was compara- 
tively easy to replace the water in a gelatin jelly with alcohol 
and this again by chloroform, turpentine, or xylol, even though 
dry gelatin does not swell in these liquids. 


PROPERTIES OF GELS 


Vapor Pressure Relations.—Freshly precipitated gelatinous 
oxides, such as the hydrous oxides of iron, chromium, aluminum, 
tin, and silicon, have a vapor pressure almost the same as water 
and maintain it until the water content of the gels is lowered 
quite appreciably. Van Bemmelen®® has examined a large 
number of such oxides and has found that the loss of water in 
dry air is continuous, the vapor pressure curve showing no breaks 
such as would be expected if definite chemical compounds— 
hydrates—were formed. Formulas for hydrates of precipitated 
oxides are frequently given in the literature, but in the vast 
majority of cases the composition indicated by these formulas is 
purely accidental, depending as it does on so many factors, such 
as the conditions of formation, the method of drying, and the 
age.°7 In general, it may be said that the metallic oxides pre- 
cipitated in a highly gelatinous form are never hydrates, so that 
they should be looked upon as hydrous oxides rather than hydrous 
hydrated oxides. This does not mean that there are no hydrates 
of the metallic oxides, for there are a few, among which may be 
mentioned Fe.03;-H,O and Al,03;.8H2O; but, as a rule, these 
must be prepared in a very special way. 

An elastic jelly, such as gelatin, loses water continuously in 
dry air, just as does a gelatinous oxide;%* but, unlike the latter, 


93 Z,. anorg. Chem., 73 (1912), 165. 

94 J. Am. Chem. Soc., 43 (1921), 1844. 

% ‘Ther den Bau quillbarer K6rper,’’ Gottingen, 1896. 

% ‘Tie Absorption,” 1910. 

97 Weiser: J. Phys. Chem., 24 (1920), 277, 505; 26 (1922), 402, 654; 
27 (1923), 501. 

% Katz: Z. Elektrochem., 18 (1911), 800. 





JELLIES AND GELATINOUS PRECIPITATES 401 


the process is very much more nearly reversible, a dry plate 
taking up moisture and swelling again in moist air. As already 
pointed out, pores are not developed by the dehydration, as in 
the case of silica gel. A still more striking difference between 
the non-elastic and elastic gels is that the former will take up a 
great deal more water when dipped in the liquid than when sus- 
pended in the vapor at the same temperature. Von Schréder?® 
studied the behavior of gelatin in liquid water and in water vapor 
and was led to conclude that the vapor pressure of water in 
gelatin must be higher than that of pure water because water 
distills from the gelatin to the vapor phase. Bancroft!°® explains 
von Schréder’s results by assuming that gelatin has a cellular 
structure. The walls of the cell will adsorb a certain amount 
of water from the saturated vapor, but the microscopic cells 
or pockets will not be filled unless the gelatin is immersed in 
water. On lifting the swollen jelly into the vapor phase, water 
will distill from the curved microscopic droplets to the plane 
surface of water in the containing vessel, because of the higher 
vapor pressure of the former. As Bancroft points out, the objec- 
tion to this explanation is that it postulates a cellular structure 
for gelatin which seems more and more improbable in the light 
of recent investigations. Wolff and Buchner!! claim that water 
does not distill from a fully distended gelatin jelly into the vapor 
phase and that von Schréder’s conclusions are the result of experi- 
mental error. Washburn'? found that moistened clays will 
dry in a closed vessel above water, a result that supports von 
Schréder’s observations; but he believed this to be due to the 
action of gravity. There seems no way of settling the question 
except by a careful repetition of von Schréder’s experiments. 

Whenever a dry gel takes up moisture, heat is evolved! and a 
contraction in volume! takes place, particularly in the earlier 
stages. Although the volume of the system water + dry gel 

% Z. physik. Chem., 45 (1903), 109. 

100 “€ Anplied Colloid Chemistry,’’ 1921, p. 75. 

101 Kon. Akad. Wet., Amsterdam, 17, May 30 (1914); Z. physik. Chem., 
89 (1915), 271. 

102 J, Am. Ceramic Soc., 1 (1918), 25. 

103 WiEDEMANN and LipEKiIne: Wied. Ann., 25 (1885), 145; RopEwaLp: 


Z. physik. Chem., 24 (1897), 193. 
104 TijpEKING: Wied. Ann., 35 (1888), 552. 


402 COLLOIDAL BEHAVIOR 


is greater than that of the swollen gel, the gel itself increases in 
volume and so may exert a very high pressure. In some experi- 
ments on dried seaweeds, Reinke! found that water was taken 
up against a pressure of 41 atmospheres, the volume increase 
amounting to 16 per cent. Similarly, Rodewald'°® found that 
starch swells against a pressure of 2,500 atmospheres. Posn- 
jack!°7 observed the amount of water with which gelatin is in 
equilibrium at various pressures and on the corresponding behay- 
ior of raw rubber in different organicsolvents. In all experiments 
the amount of liquid taken up decreases with increasing pressure. 
The data do not enable us to determine what pressure would be 
necessary to prevent any swelling or to remove all the absorbed 
liquid from a swollen jelly; these values would probably be very 
high in every case. Some idea of the magnitude of the swelling 
pressures of gelatin may be obtained by coating a glass plate 
with gelatin which has absorbed the maximum amount of water 
and observing the degree to which the glass plate is bent by the 
drying film of gelatin.°* The strain is frequently sufficient to 
break the plate or to pull pieces of glass off the surface. 
Elasticity—The elasticity of gelatin gel has been investi- 
gated most extensively since it may be readily molded 
into any desired shape and comparable samples are fairly easily 
obtained. ‘The values of the modulus of elasticity obtained by 
different investigators'°® agree fairly well considering the ditffer- 
ences in method of procedure. Leick obtained values that range 
from 2.42 g. per mm.” for a 10 per cent jelly to 29.4 for a 45 per 
cent jelly. The modulus was found to be roughly proportional 
to the square of the concentration. The value varies with the 
load and is not constant until after a 24-hour period of setting. 
Poission’s ratio—the ratio of the relative contraction of the diam- 
eter to the relative change in length—has been determined 
repeatedly!" for gelatin and found to be 0.5, showing that there 


105 Hanstein’s botan. Abhandl., 4 (1879), 1. 

106 JZ. physik. Chem., 24 (1897), 193. 

107 Kolloidchem. Bethefte, 3 (1912), 417. 

108 GRaHAM: J. Chem. Soc., 17 (1864), 320. 

109 Maurer: Wied. Ann., 28 (1866), 628; Fraas: Ibid., 53 (1894), 1074; 
Leick: Drude’s Ann., 14 (1904), 139. 

110 Maurer: Loc. cit.; Fraas: Loc. cit.; Lurcx: Loc. cit.; BsERKEN: Wied. 
Ann., 43 (1891), 817. . 





JELLIES AND GELATINOUS PRECIPITATES —— 403 


is no actual change in volume when a gel undergoes extension. 
The elasticity modulus of gelatin is influenced by the presence 
of various substances added to the gel, just as these affect gelation. 
Thus, Leick finds that chlorides lower the value just as they 
lower the viscosity and setting temperature of the sol, while 
glycerin and cane sugar have the opposite effect on each of the 
constants. 

The elasticity of gelatin gels is perfect only for small loads 
applied for a short time; yet but little work has been done on 
the relaxation time in such systems. Rankine!!! maintained 
3.5 to 4.5 per cent jellies at constant strain and plotted the stress 
necessary to do this against time. With the concentrations of 
jelly he employed, the stress never became zero. Reiger}!? 
determined the relaxation time optically by observing the dis- 
appearance of double refraction produced by strain. He found 
a relaxation time of 10 minutes for a 20 per cent jelly and of 41 
minutes for a 40 per cent jelly at a temperature of 29°. The 
conditions are different at lower temperatures, as observed by 
Hatschek.!!? The latter bent a rectangular prism of a 10 per cent 
jelly and held it at 15° for 5 days until the stress disappeared 
and the jelly could be moved without straightening itself appre- 
ciably. Photographs taken in polarized light, when the stress 
was first applied, and at the end of the 5-day interval were prac- 
tically identical. The optical anisotropy caused by strain 
did not disappear even after the external stress was removed. 
Similar observations were made by Harrison!" with india rubber, 
cotton, wool, silk, and other fibers and by Auerbach! with 
cotton and woolen threads. Bradford!!® believes that gel sub- 
stance deposits on the larger particles of a jelly on standing. Ifa 
jelly is held under a strain, the particles will be cemented in 
their constrained position and so cannot move on removal of 
the stress. The persistence of the optical appearance in polarized 
light is thus attributed to the internal mechanical strain. 


111 Phil, Mag., 12 (1906), p. 447. 

112 Physik. Z., 2 (1901), 213. 

113 Loc. cit., Ref. No. 53, p. 37. 

114 Proc. Roy. Soc., A 94 (1918), 460. 
115 Kolloid-Z., 32 (1923), 369. 

116 Toc. cit., Ref, No, 53, p. 56. 


404 COLLOIDAL BEHAVIOR 


As has been pointed out, the elasticity modulus of a gelatin 
jelly increases approximately as the square of the concentration 
and the relaxation time likewise increases rapidly with increasing 
concentration of gel. Since the product of elastic modulus and 
relaxation time is viscosity, it follows that the viscosity must show 
a marked increase with increasing concentration. 

A gelatin jelly becomes warmer when it is stretched and cooler 
when compressed, just as rubber does. ‘The compressibility!’ 
of gelatin is 10 X 10~°, which is approximately ten times greater 
than that of solids at ordinary temperatures. ‘The compressi- 
bility increases with rising temperature and when the jelly lique- 
fies it becomes 48 X 10~*, the value for water. 

Non-elastic jellies may vibrate like a rigid body under certain 
conditions. This phenomenon has been investigated by Holmes, 
Nicholas, and Kauffman.'!® Some jellies were obtained in a 
glass test tube that gave a tone two octaves above middle C. 
It was demonstrated that the vibrations were not longitudinal 
but were transverse, the vibration frequency varying approxi- 
mately inversely as the diameter of the cylinder of jelly. If the 
jellies were prevented from touching the walls of the glass tube 
by coating the latter with vaseline, the vibration frequency was 
much lower than for similar jellies adhering to the walls. As 
ordinarily prepared, the jellies are in a state of strain or tension 
and this affects the pitch of the tones they emit. The vibration 
frequency is increased by increasing the concentration of silicic 
acid and by the presence of excess mineral acid, factors that 
increase the tension and thus the effective rigidity. The same 
factors that increase the vibration frequency increase the synere- 
sis of the jelly, thus showing that both vibration and syneresis 
have a direct relation to tension. 

Optical Properties.—In the preceding section reference has been 
made to the double refraction produced in a jelly when subjected 
to mechanical deformation. This property is possessed by very 
dilute sols of gelatin, gum, collodion, etc., as well as by jellies. 
It does not occur, however, in solutions of glycerol and gelatin 


117 Barus: J. Am. Chem. Soc., 6 (1898), 285. 

118 J, Am. Chem. Soc., 41 (1919), 13829; Cf. Kontrauscu: Z. physik. Chem., 
12 (1893), 773; HatrscuEex: “Introduction to the Physics and Chemistry 
of Colloids,’”’ 1916, p. 55. 


W-28:-- 


JELLIES AND GELATINOUS PRECIPITATES 405 


which are more viscous than the sols, and so cannot be due to 
ordinary viscosity. There appears to be no satisfactory explana- 
tion of the ultimate cause of the optical phenomenon but it 
seems reasonable to assume that the appearance of strained 
elastic sols and gels in polarized light is due to the effect of the 
stress on the elastic solid or semi-solid phase. This view seems 
to accord with the observations on gelatin jellies by Leick,!1 
who found that the double refraction is approximately propor- 
tional to the concentration of the jelly and approximately pro- 
portional to the strain. Salts affect the double refraction in 
much the same way as they influence the modulus of elasticity. 

The double refraction in sols and jellies subjected to strain 
should be distinguished from the phenomenon that frequently 
occurs in sols, even when no external stress is applied. Thus, 
Freundlich, Schuster, and Zocher!”° find that double refraction 
occurs on cooling down a solution of benzopurpurine. This is 
attributed to the development of long particles as a result of 
ordered coagulation, which must be differentiated from the 
usual unordered coagulation. Similarly, Humphrey'*! observed 
double refraction as a result of motion in a vanadium pentoxide 
sol. In this case the phenomenon is assumed to result from the 
formation of rod-shaped particles which set themselves in a 
definite direction when the liquid is caused to move. 

Gelatin in solution possesses the property of mutarotation. 
Smith!*? has shown that the specific rotatory power of a 3 per 
cent commercial gelatin figured on a moisture-and-ash-free 
basis is —141° at 35°C. and —313° at 15°C. The property is 
attributed to a thermo-reversible equilibrium that may be 
represented thus: Sol from A@gel from B. This. reaction 
appears to be bimolecular. The increase in levorotation on 
lowering the temperature, which signifies increasing formation 
of the gel form B, closely parallels increase in viscosity. Sul- 
fates displace the equilibrium toward the gel side between 15° 
and 35°, whereas chlorides, bromides, and iodides lower the 
viscosity and shift the equilibrium toward the sol side. 


119 Drude’s Ann., 14 (1904), 139. 

120 7. mhysik. Chem., 105 (1923), 119. 

121 Proc. Phys. Soc., London, 35 (1923), 217. 

122 J. Am. Chem. Soc., 41 (1919), 135; Cf. Auexanper: ‘‘Glue and Gela- 
tin.” 1923, p. 47. : 


406 COLLOIDAL BEHAVIOR 


The refractive index of both gelatin sols and gels has been 
shown by Walpole!** to be a linear function of the gelatin con- 
centration. As previously mentioned, the curve obtained by 
plotting refractive index against temperature shows no discon- 
tinuity when the sol is transformed into a jelly. This indicates 
that the transformation from sol to jelly is a gradual and regular 
process, such as would be expected if the transition was a conse- 
_ quence of the linking up into loose aggregates of particles already 
existing. 

Diffusion.—The early investigations of Graham?" led him to 
conclude that the rate of diffusion of salts in a gelatin jelly is 
the same asin pure water. But later work of Nell!2° and Bechold 
and Ziegler!*® showed that the resistance to diffusion is negligible 
only in case the gels are quite dilute, and Demanski!”’ found that 
the conductance of a gelatin solution is always less than that of 
pure water. These observations are in accord with what one 
might expect if the structure of the jelly is an enmeshing network 
of hydrous particles. A thin mesh will have but little effect 
on the rate of diffusion of an electrolyte, whereas a thick mesh 
will tend to retard the process. The rate of diffusion of a sol 
into a jelly will be very slow indeed on account of the resistance 
offered by the network to the passage of particles of colloidal 
dimensions. Bechold'?* found that hardened gelatin could be 
employed for ultrafiltration, since the network structure holds 
back colloidal particles while allowing solutes to pass through, 
just as collodion filters do. 

The presence of certain solutes in a jelly may have an influence 
on the rate of diffusion of other solutes. Thus, Bechold and 
Ziegler'*® found that NaCl and Nal had little effect, whereas 
Na2SO., dextrose, glycerin, and alcohol reduced the rate of 
diffusion of certain solutes. There seems to be a qualitative 
parallelism between the effect of solutes on the elastic modulus 


123 Kolloid-Z., 18 (1913), 241. 

124 Trebig’s Ann., 121 (1862), 5, 29. 
12 Drude’s Ann., 18 (1905), 323. 

126 Z. physik. Chem., 56 (1906), 105. 
127 Z. physik. Chem., 60 (1907), 553. 
18 Z. physik. Chem., 60 (1907), 257. 
£29 10G), Cle 


JELLIES AND GELATINOUS PRECIPITATES 407 


and the rate of diffusion in the sense that substances which 
increase the former reduce the latter.1#° 

A study of chemical reactions in jellies is made possible by the 
ease with which electrolytes diffuse in such media. The usual 
method of procedure consists in adding one electrolyte to a sol, 
which is allowed to set, after which a solution of a second elec- 
trolyte is poured on the jelly and allowed to diffuse into the mass, 
where interaction takes place. If a crystalline precipitate is 
formed by the reaction, the crystals will be much larger and 
well formed than if the solutions are mixed directly. By this 
procedure, Hatschek and Simon!*! prepared gold crystals in a 
silicic acid jelly by reducing gold salts with a number of reducing 
agents. Hatschek!** also prepared fairly large crystals of several 
insoluble salts in gelatin and silicic acid jellies. Hatschek’s 
work has been confirmed and extended by a number of investi- 
gators, particularly by Holmes!*? who prepared magnificent 
crystals of several metals and salts in silicic acid jellies. The 
function of the jelly is to prevent rapid mixing of the interacting 
solutions, thus avoiding rapid precipitation and the consequent 
formation of amorphous particles or small crystals. 

Under certain conditions, reactions in jellies carried out as 
described above lead to the formation of rhythmic bands of 
precipitates instead of large crystals. This interesting phenom- 
enon was first noted by Liesgang,!*4 who obtained rhythmic rings 
of silver chromate when a drop of silver nitrate was placed on a 
‘gelatin film containing dilute potassium chromate. Ostwald 
explained the formation of the rings in Liesgang’s experiment by 
postulating that the silver chromate forms a supersaturated 
solution which diffuses along with the silver nitrate until the 
metastable limit is reached, when it precipitates. By repetition 
of the process, alternate gaps and bands are _ produced. 
Bechold!** believes that the precipitate which constitutes the 


130 FurTH and BuBANovic: Biochem. Z., 90 (1918), 265; 92 (1918), 139; 
Stites: Biochem. J., 14 (1920), 58. 

131 J, Soc. Chem. Ind., 31 (1912), 439; Mining fon World, 37 (1912), 280. 

132 Kolloid-Z., 10 (1912), ‘itp 

Men SL ys. chen. 21 (1917), 709. 

134 Tiesgang’s phot. Archiv. (1896), p. 321. 

135 Z, physik. Chem., 52 (1905), 185. 


408 COLLOIDAL BEHAVIOR 


bands is slightly soluble in the reaction products and hence that 
new bands can form only after a point is reached where the con- 
centration of the reaction products is sufficiently small. Brad- 
ford'*® suggests that one of the reacting solutes is adsorbed by 
the growing precipitate, thus giving zones which are practically 
free from it. Holmes**” attributes the phenomenon to the con- 
ditions affecting the rate of diffusion. He points out that, 
according to Fick’s law of diffusion, the rate of diffusion is greatest 
where the difference in concentration of the ions in question in 
two contiguous layers is greatest, that is, just below the front of 
a precipitation band. As a result, the region near the band 
decreases in concentration of negative ion, for example, faster than 
does the space below. Finally, the positive ions have to advance 
some distance beyond the band to find such a concentration of 
negative ions that the solubility product of the salt is exceeded 
and precipitation occurs with the formation of a new band. 

The number and variety of the hypotheses that have been 
offered to account for the formation of Liesgang rings indicate 
that the process is not a simple one. Since none of the theories 
that have been proposed seem to take all of the factors into 
consideration, there is as yet no entirely satisfactory explanation 
of the phenomenon. Not the least of the objections that have 
been offered to the hypotheses of Ostwald, Bechold, and Bradford 
is that the specific effect of the jelly is left out of account. That 
the nature of the jelly does play an important réle in certain 
cases is evidenced by the fact that rhythmic bands of certain - 
salts will be formed only in certain jellies. Thus, silver chromate 
forms bands in gelatin jellies but not in agar; lead chromate forms 
bands in agar but not in gelatin; while neither silver nor lead chro- 
mate forms bands in silicic acid jelly, although copper chromate 
does. It seems, however, that investigators have placed rather 
too much emphasis on the specific effect of the jelly, since rhyth- 
mic bands may be prepared without any jelly at all. Thus, 
Holmes!** prepared silver dichromate bands in loosely packed 
flowers of sulfur and Chapin?** secured distinct bands of ammo- 


136 Biochem. J., 10 (1905), 169. 

137 J. Am. Chem. Soc., 40 (1918), 1187. 

138 J. Am. Chem. Soc., 40 (1918), 1194. ' 

139 C'f. ‘Exercises in Second Year College Chemistry,” 1922, p. 8. 


JHLLIES AND GELATINOUS PRECIPITATES 409 


nium chloride by allowing hydrogen chloride and ammonia to 
diffuse through the air from opposite ends of a tube over 1 m. in 
length. Holmes’ diffusion hypothesis seems to be an important 
step toward the solution of the problem of rhythmic banding. 
It should be pointed out, however, that a great deal of work 
remains to be done before such an important factor for banding 
as the permeability of the jelly can be accurately formulated. 


CHAPTER XVI 


THE STUDY OF SOAP SOLUTIONS AND ITS BEARING 
UPON COLLOID CHEMISTRY 


By 
JAMES W. McBAIN 


The phenomena met with in the study of soaps are so manifold 
that it is only by exercise of the severest restriction that a clear, 
general picture can be obtained. On the other hand, they 
lend themselves particularly to the study of the colloidal 
condition and its relation to other states. Soaps are of known 
chemical composition, and constitute in themselves so num- 
erous a family that almost any desired combination of prop- 
erties can be obtained. Furthermore, they are typical of a 
very great group of ionizable materials and they exhibit a partic- 
ularly close parallelism with the behavior of the salts of protein 
and gelatin. It has been possible to obtain from these soap 
solutions unusually definite results, which are of general signi- 
ficance for the understanding of colloids and colloid chemistry. 
The quantitative results also serve to link together the behavior 
of colloids with that of ordinary crystalloids. 

Soap occurs in five chief forms, the first three of which are 
properly regarded as solutions, whereas the last two are closely 
related to a crystallized condition. Whenever the whole of the 
soap is in either true or colloidal solution, itis clear and transparent 
or only slightly opalescent. Three such transparent states occur: 
fluid sol, clear elastic jelly, and clear plastic anisotropic liquid 
(crystalline liquid). All three are important, the first two on 
account of their bearing upon the theory of colloids and the 
structure of jellies, and the third, which still needs much investi- 
gation, because of its essential rédle in the processes of soap 
boiling. The detergent action of soaps is practically confined 
to the first form. The fourth and fifth forms in which soaps 

410 


THE STUDY OF SOAP SOLUTIONS 411 


may occur are as curds and as true crystals (compare photo- 
micrographs, figures 3-7). In the case of the potassium or soft 
soaps, the crystalline condition is the most stable, whereas 
sodium soaps are usually opaque masses consisting of white curd 
fibers, which very often enmesh a saturated soap solution of 
one of the first three forms. A bar of good toilet soap 
consists almost exclusively of these extremely fine curd fibers. 
The parallel arrangement of curd fibers accounts for the 
‘feather’? of good household soap. Although many sodium 
soaps may be temporarily obtained in a crystalline form, these 
crystals are unfortunately less stable than the curd fibers and 
are soon replaced by them. 

It will clear the ground to state that dilute soap solutions are 
ordinary crystalloids, whereas more concentrated ones are 
colloidal electrolytes; and that excess of soap above saturation 
value separates out in the form of true crystals in the case of 
potassium soaps and of white curd fibers in the case of sodium 
soaps. The transparent jellies are confined to a portion of the 
region in which the soap exists as colloidal electrolyte. The most 
concentrated solutions, or those to which much salt has been 
added, are those which constitute anisotropic liquids; and they 
are immiscible with the other forms of soap. 


Soap SOLS AND THE THEORY OF COLLOIDAL ELECTROLYTES 


In the first place, it has been shown that hydrolysis is of 
negligible significance except in very dilute solution. Indeed, 
the hydrogen soap, cetyl sulfonic acid, in which hydrolysis 
is impossible, behaves in most respects exactly like potassium 
stearate. Hydroxyl ion is a minor constituent of ordinary soap 
solutions, being present only to the extent of about 0.001N. 
This has been shown by measurements of: 

1. Electromotive force, hydrogen electrode. 

2. Catalysis, nitrosotriacetonamine. 

3. Conductivity of solutions in contact with insoluble acid 
soaps. 

4, Analysis of the ultrafiltrate of concentrated soap solutions, 
where nearly all the soap is held back as colloid. 


412 COLLOIDAL BEHAVIOR 


5. Indicators. 

6. Electrolytic migration. 

We conclude, therefore, with some confidence that the major 
properties of soap solutions are due to the soap itself and not 
to products of hydrolysis. There are, then, two properties of 
soap solutions of great significance; namely, osmotic activity and 
conductivity. 

The osmotic activity was unexpectedly difficult to measure 
and the well-known work of Krafft and Smits was found to be 
invalid. However, a wide range of concentrations of many soap 
solutions has been studied by the following four methods: 

1. Lowering of freezing point. 

2. Lowering of dew point. 

3. Minimum pressure for ultrafiltration through a dense 
membrane. 

4. Lowering of vapor pressure. 

The result is that a strong solution of a higher soap may be 
said to exhibit one-half the osmotic activity of an ordinary salt. 
Of course, this is much increased in dilute solution and tends to 
diminish with rise of temperature, and the various soap solutions 
exhibit every intermediate value in the most regular fashion. 

The conductivity is equal to that of a salt. This is attested 
by hundreds of measurements in which the most careful search 
has been made for sources of chemical or physical error, and 
results obtained in other laboratories have confirmed the 
writer’s researches. 

There seems to be no alternative to the conclusion that half the 
conductivity of a soap solution is due to a negative carrier which 
does not exhibit appreciable osmotic activity and is, therefore, 
colloidal; that is, the ionic micelle. A colloidal electrolyte is a 
salt in which one of the ions is replaced by ionic micelles, that is, 
highly charged and solvated colloidal particles. The undisso- 
ciated colloidal electrolyte consists of neutral micelles; the ionic 
micelles essentially of aggregated and hydrated fatty ions. 

This interpretation leads to definite values for the con- 
centration of each of the constituents of the solution, such 
as sodium ion and ionic micelle; and the mobility and equivalent 
conductivity of the latter follow directly. These values have 
been confirmed by the following independent methods: 


THE STUDY OF SOAP SOLUTIONS 413 


1. Measurement of sodium and potassium ions by sodium and 
potassium electrodes. 

2. Ultrafiltration through membranes which hold back all the 
colloid and permit all the crystalloid to pass. 

3. Measurements of electrolytic migration. 

This view of the constitution of soap solutions is also in har- 
mony with a mass of qualitative information. <A typical diagram 
showing the relative proportions of the constituents is that here 
given for sodium palmitate (Fig. 1). 


100 








60} 
: ae 


RSI 
tt 3S aa 








Relative Proportions of Constituants, per cent 





= 


Q oN a 2K IN OSN Oe O4N iN O5N Ol OGN OTN IN. OBN 0: 0.9N N ION LIN L2N GN 14N ISN ION 
Total Concentration of Solution (Nw) 


Fie. 1—The relative proportions of the various constituents of solutions of 
pure sodium palmitate at 90°. (The asterisk marks the field showing the pro- 
portion of acid soap. 2NaP.HP, present.) 


Nearly all the ordinary soaps have been studied under a variety 
of conditions, and it is found that there is but little difference in 
the amount of neutral micelle in corresponding sodium and potas- 
sium salts. On the other hand, in potassium soap solutions, 
which are always rather more dissociated than the corresponding 
sodium soaps, the proportion of ionic micelles is always about 
five-fourths greater than in the sodium soap, and, moreover, 
the ionic micelles in potassium soaps persists to a dilution about 
twice as great as in the solutions of sodium soaps. Although 
the ionic micelle must contain a certain amount of neutral colloid, 


414 COLLOIDAL BEHAVIOR 


there is evidence from ultrafiltration and from migration data 
that most of the latter exists independently as neutral micelles. 

The chief difference between potassium and sodium soaps, 
and also between the saturated and unsaturated homologous 
series, is not primarily due to differences in the constitution 
of their solutions, but rather to the great difference in solubility, 
the highest saturated sodium soaps being practically insoluble 
at room temperature, although all are extremely soluble at the 
boiling point. Dilute solutions are crystalloidal, apart from the 
colloidal acid soaps arising from hydrolysis, which is prominent 
in extreme dilution. Formation of colloidal electrolyte is 
favored by lowering of temperature, increase in molecular weight 
in the homologous series, and addition of salts. 


THEORETICAL CONSEQUENCES 


1. It is thus established that true, complete, reversible equili- 
brium can subsist in a colloidal system between all the various 
crystalloidal and colloidal constituents. 

2. There is here a clear proof that the colloidal particles are 
definite stable aggregates in true equilibrium with their crystal- 
loidal components. The unit is the colloidal aggregate, not the 
molecule or simple ion, and the aggregate is large enough to be 
held back by an ultrafilter which allows the molecule to pass 
through. This throws decidedly unfavorable light upon the 
recent tendency to assume that the colloidal particles in the closely 
parallel protein and gelatin systems are merely large molecules 
which are otherwise in true solution and not aggregated. In the - 
case of soap, each particle of ionic micelle is probably built up 
around 10 ions which have retained their respective electrical 
charges. 

3. Colloidal particles may conduct much better than true ions. 
This was our initial suggestion for harmonizing the data for 
protein systems. But it is necessary to be clear that it is not 
mere electrophoresis which is meant. The actual conductivity of 
one ionic micelle containing, say, 10 palmitate ions is 20 or 30 
times greater than that of a single palmitate ion; so that the 
equivalent conductivity of the ionic micelle is still several times 
that of the simple fatty ion. This is in harmony with the 
principle underlying Stokes’ law. 


THE STUDY OF SOAP SOLUTIONS . 415 


On the other hand, the osmotic activity of a given number 
of fatty ions largely disappears when they unite to form a single 
particle of ionic micelle. It is the usual view of colloid chemists, 
based on such observations as those of Svedberg and Perrin, 
that a colloidal particle behaves in this respect like a single mole- 
cule, so that if 10 ions go to form 1 ionic micelle its osmotic 
activity would be diminished tenfold. 

4, Consideration of the relationship between a particle of ionic 
micelle, with its high conductivity, and the corresponding 
sodium or potassium ions, leads to a modified picture of 
the electrical double layer and the phenomena connected there- 
with. It is obvious that the electrical conductivity of these 
ions and charged surfaces is not impaired by their participation 
in an electrical double layer. This will be further discussed 
under a later heading. 

5. The discovery of the ionic micelle, the extreme type of a 
fully charged colloidal particle, brings out clearly the transition 
through the slightly charged suspensoids to the other extreme of 
neutral colloids, such as rubber in benzene or nitrocellulose in 
non-lonizing solvents. It seems clear that the existence and 
stability of such neutral colloids may be almost or quite indepen- 
dent of electrical charges, and that either solvation or sorption 
of solvent.is the key to their behavior. 

There would seem to be some connection between the influence 
of solvation in stabilizing a particle and the very general rule 
that mutually soluble substances are chemically similar. The 
effect of the solvent in disintegrating the solid is the same in 
both cases. 

6. It has been shown that soap in colloidal solution is hydrated, 
and the method used is of general applicability in colloid 
chemistry. It consists in the ultrafiltration of a sol to which an 
indifferent reference substance has been added. The filtrate is 
more concentrated as regards reference substance than the 
original sol, and from this the amount of solvent held back in 
combination with the colloid is readily calculated. Similar 
values have been obtained from electrolytic migration. 

7. In Fig. 2 is shown what appears to be a rational classifica- 
tion of all solutions, showing the gradual transition between 
each pair of neighboring types. 


416 COLLOIDAL BEHAVIOR 


The connection between suspensoids and colloidal electrolytes 
calls for furtherelucidation. The transition which isinsisted upon 
here is the transition in behavior conditioned by the respective 
proportion of electrical charges. In discussing suspensoids we 
freely refer to their electrical charges, but almost invariably ignore 
the equal number of ions of opposite sign which must inevitably 


CRYSTALLOID 


SUCROSE 
SEMI-COLLOID WEANE ELECIRGLIIE 
DEXTRINE HgCle 
NEUTRAL COLLOID ' ELECTRO 
STARCH KCl 


SUSPENSOID COLLOIDAL ELECTROLYTE 
As, 5; OO SOAP 


Fic. 2.—A rational classification of solutions, showing the gradual transition 
between each pair of neighboring types. 


accompany them. From the standpoint of the solvent, the 
properties of a particle do not greatly depend upon whether 
the particle consists of a chemical compound, whether a single 
molecule or polymer or aggregate of molecules, or whether the 
particle consists of a solid kernel only the surface of which has 
been brought into relationship with the constituents of the 
solution and the electrical charges of which are, for instance, the 
result of adsorption on its surface. 

It will be shown under a later heading that the transition 
between suspensoid through colloidal electrolyte to electrolyte is, 
from the standpoint of the behavior in an electrical field, not 
merely formal but inherent. 

Again, from the standpoint of osmotic behavior, it seems that 
we have still to realize the full implications of the dictum that a 
colloidal particle behaves as a single molecule. It would provide 
a plausible reconciliation between the attempts to apply mass law 
and phase rule respectively to particles of colloidal dimensions. 
Both describe the equilibrium between particles and the corre- 


THE STUDY OF SOAP SOLUTIONS 417 


sponding single molecules in the solution. For instance, the 
mass law might be applied to crystalline particles in the form: 


Ale A. 
or, in general 
N 


(A,)* = K,.(A)* 


Concentrations are measured in numbers of particles per unit 
volume and the chief factor to take into account is the change 
in number of particles per unit volume and not the actual size 
of the particles. It is only with the most minute particles of 
practically molecular dimensions that the value of K, should 
reflect any specific tendency to form definite polymers and this 
factor should be appreciably constant as soon as n has attained, 
say, two significant figures. Assuming K, independent of the 
size of the particles if a large particle of one gram mol (con- 
taining N molecules) is cut into N/n particles the number of 
particles will be increased N/n fold and the concentration of 
single molecules would have to be increased by n1/N/n fold 
to balance the equilibrium. ‘Taking numerical examples the 
effect would be about 1/1000th of a per cent for particles 0.1u 
diameter and about one per cent for particles of 10uu and of 
the order of magnitude of 100,000 fold for lum diameters. 
From this hypothesis it would follow that the concentration of 
single molecules A would be constant to an exceedingly high 
degree of accuracy as soon as the aggregates, whether solid or 
liquid, became of visible dimensions, since the number of mole- 
cules in 1 g. molecule is 60 & 107. On the other hand, solubility 
and vapor pressure would increase, at first extremely slowly, then 
rapidly within the region of colloidal dimensions, until, in the 
special case of molecules so large as to be colloidal, their effective 
concentration would become identical with the number of parti- 
cles. The principle here very tentatively made explicit should 
serve as a useful approximate guide in this field, even if the 
deviations from the mass law amounted to many fold, since the 
range of aggregation here contemplated is so enormous (10*4 
fold). It predicts a variation, such as that observed by Hulett 
and others, in the solubility and vapor pressure with grain size, 


418 COLLOIDAL BEHAVIOR 


although apparently with far finer particles. The phase rule 
would thus appear in the guise of a special form of the mass law 
for coarse dimensions, and it provides a mechanism for Bancroft’s 
recent statement that particles, if fine enough, behave as if they 
were in true solution. Further, it would explain the fact that 
particles of colloidal dimensions persist indefinitely since their 
solubility, while still within colloidal range, becomes practi- 
cally identical with that of massive material. 


INDEPENDENCE OF CLASSICAL DISSOCIATION THEORY 


It is essentially for the sake of convenience that the 
results have been discussed in terms of the Arrhenius dissociation 
theory. ‘The same conclusions follow if the conceptions of the 
100 per cent ionization hypotheses are employed. Practically 
only the interpretation of the crystalloidal soap is affected, 
since now the crystalloidal part of soap might be considered 100 
per cent ionized. To a very large extent, although not entirely, 
the Arrhenius and the 100 per cent theories are interchangeable 
methods of describing the undisputed facts of ordinary electro- 
lytes. Whichever theory is employed, the striking fact is at once 
encountered that, in spite of the high conductivity, soap exhibits 
far smaller osmotic effects than that expected of the whole 
group of uni-univalent electrolytes, such as potassium chloride. 
This corresponds with cations of ordinary, and anions of negli- 
gible, activity. Low activity is characteristic of high valency— 
the ionic micelle. Direct experimental confirmation is obtained 
from experiments on ultrafiltration. It should be remembered, 
too, that the 100 per cent ionization hypotheses are admittedly 
inapplicable to weak or complex electrolytes, such as, for example, 
cadmium iodide where, during electrolysis, cadmium migrates 
towards the anode. Even crystalloidal soap in dilute solution is 
by no means as strong an electrolyte as any of the ordinary 
uni-univalent salts. 


COLLOIDAL ELECTROLYTES 


There is evidence that colloidal electrolytes constitute an 
enormous class of substances, probably as numerous as acids 
and bases put together, occurring both in aqueous and non- 


THE STUDY OF SOAP SOLUTIONS 419 


aqueous solutions. Indeed, they would appear to be particularly 
important in non-aqueous solutions, although this probability 
has so far been entirely neglected in the published literature. 
Silver bromate and diethylamine constitute a good example, 
which the writer has investigated. The characteristics of this 
class are essentially the occurrence of polymolecular aggregates 
and ionic micelles. 

Some of the very numerous substances which must be recog- 
nized as belonging to this group are the proteins and gelatin 
salts, dyes, indicators, the higher sulfonic acids and hydro- 
chlorides, silicates, telurates, and many inorganic substances— 
in fact, most substances of high molecular weight or containing 
long carbon chains which are capable of splitting off an ordinary 
ion. | 

Congo red was simultaneously investigated by Donnan and 
Bayliss. The work of Donnan and Harris,'! in agreement with 
that of Bayliss? and Biltz and Vegesack,* shows that the osmotic 
pressure of Congo red is considerable, but does not exceed that 
of a simple non-electrolyte. On the other hand, the conductivity 
was unexpectedly high, being nearly equal to that of sodium 
chloride. Donnan’s interest was chiefly attracted to the impor- 
tant theory of membrane equilibria, which he thought might be 
sufficient explanation of the discrepancy, since the sodium ions 
were not expected to contribute in the ordinary way to the os- 
motic pressure. He discussed the formation of high complexes 
such as (Na:R)o@20Na + (NaR)53, but added, “it is difficult 
to reconcile such an assumption with the high values obtained for 
the molar conductivities.” 

On the other hand, Bayliss writes, after making various alterna- 
tive suggestions: 


The possibility of aggregated simple ions carrying the sum of the 
charges of their components is suggested in order to explain the experi- 
mental results. .. . These large organic ions may aggregate 
while retaining the combined charges of their components. This 
seems to imply, however, so great a density of the charge on the surface 
of the aggregates as to be improbable. . . . The difficulty as to 

1 Trans. Chem. Soc., 99 (1911), 1554. 


2 Proc. Roy. Soc., B, 84 (1911), 229 and earlier papers. 
3 Z. physik. Chem., 73 (1910), 481; 77 (1911), 91, ete. 


420 COLLOIDAL BEHAVIOR 


the large charge on the aggregated anion remains. . . . It may 
be tentatively suggested that aggregation of molecules may play some 
part in the mode of ionization. 


The existing data for other dyes and for indicators are of a 
similar nature and may be similarly accounted for. Many of the 
unexplained reactions of indicators, particularly with reference 
to the sometimes enormous effect of neutral salts, are now seen 
to be probably due to disturbance of the equilibria between 
colloid and crystalloid and, hence, between the various possible 
crystalloidal forms of the indicator radical. We have proved 
that such true equilibria occur, and that every change in the 
colloid determines a corresponding alteration in the position 
of equilibrium. 

Miss Norris‘ has in this laboratory compared the properties 
of cetyl sulfonic acid (C16H3;5O03H) with those of the soaps. The 
properties and the equilibrium diagram are found to bear a 
detailed resemblance to the behavior of the higher soaps, such 
as potassium stearate. 

Another group of sulfonic acids prepared by Sandqvist® 
‘are the derivatives of 10 bromo- and 10 chloro-phenanthrene: 


Br 


With the Br at 10 and sulfonic acid group at 6, the behavior 
is apparently identical with that of soaps. In dilute solution the 
acid behaves like an ordinary electrolyte. With increase in 
concentration the solution becomes more and more colloidal in 
character and ultimately the isotropic solution becomes aniso- 
tropic. Decreased temperature has the same effect as increase 
of concentration. Addition of strong acids produces much the 
same effect as increase of concentration. Viscosity increases very 
rapidly with concentration but the electrical conductivity remains 
approximately normal, even when the viscosity has become 
very large. Lowering of freezing point is deficient in more 

* Trans. Chem. Soc., 121 (1922), 2161. 


® Ber., 48 (1915), 2054; Kolloid Z., 19, (1916), 113; J. Chem. Soc., 110 
(1916), 556; Ann., 417 (1918), 1; 17. 


THE STUDY OF SOAP SOLUTIONS 421 


concentrated solution. The corresponding chloro-derivative is 
apparently much more colloidal, as judged by its much greater 
viscosity, and is also much more anisotropic. The 10 bromo 3 
sulfonate forms yellow flocculent solutions. 

This brings us to the great influence of the shape of the molecule 
on the capacity for forming colloidal electrolytes and anisotropic 
liquids. The soaps are all long-chain compounds. Of two 
similar organic compounds, it is the long molecule which is colloi- 
dal, while the more rounded or symmetrical form is much less so 
or not at all. For instance, in Sandqvist’s compounds, if the 
bromine is replaced by a second sulfonic acid, the solutions are 
neither colloidal nor anisotropic. Or again, if two molecules 
of the bromo sulfonic acid become linked together in the 10 
position through oxidation and elimination of bromine, the 
viscosity is almost completely lost. Similar observations have 
been made when comparing the sodium salts of a and 8 naph- 
thalene sulfonates. °® 


THE EXISTENCE AND PROPERTIES OF SOAP JELLIES 


The salient facts observed by Miss Laing are the following: 
The conductivity of a soap solution is quantitatively unaffected 
when it sets to a clear, transparent, elastic gel. The same is 
true of its osmotic activity, as measured by lowering of vapor 
pressure, and of the concentration of sodium ions measured by 
the sodium electrode. Indeed, the only difference between sol 
and gel appears to be the mechanical rigidity and elasticity of the 
latter. Neither shows any structure in the ultramicroscope. 
During electrolysis the relative movement of the constituents of 
the solution is independent of whether it is a clear, elastic jelly 
or a fluid sol. 


THE STRUCTURE OF JELLIES 


1. The identity of the significant properties of sol and gel 
proves that the same equilibria exist in each. The colloidal 
particles in sol and gel must be identical in kind and amount. 


6 Trans. Chem. Soc., 121 (1922), 2167. 


422 COLLOIDAL BEHAVIOR 


2. The unchanged conductivity shows that the process of 
electrical conduction is unchanged in kind and degree. 

3. The identity of conductivity of sol and gel is particularly 
significant in discussing the structure of gels. Any theory 
conflicting with this is evidently untenable, as, for instance, Wo. 
Ostwald’s and M. H. Fischer’s suggestion that gelatinization is 
due to reversal of phases; similarly, with a closed honeycombed 
structure, or even an only partly open structure. Some micellar 
theory of gels is clearly indicated. In the case of soaps the 
structure is probably filamentous. 





Fic. 3.—1.0 n Sodium laurate. Curd fibers appearing during cooling. (Mag- 
nification 600 diameters.) 


4. Thus, although soaps, proteins, etc. are commonly called 
emulsoid colloids, they cannot be regarded as emulsions. 

5. Once more, there is strong evidence that the colloidal unit 
is an aggregate of molecules, and that these colloidal units per- 
sist and retain their identity in larger structures, such as gels 
and coagula. This behavior appears to be general; compare, for 
instance, the study of particulate clouds (aerosols). 

6. The apparent viscosity of such systems as soap and gelatin 
solutions has no marked effect upon electrical conductivity; the 


Pe ee he ee ee ee ee Pe ee 


THE STUDY OF SOAP SOLUTIONS 423 


viscosity of a soap sol may be altered a thousand fold, or it may 
be solidified to a clear, elastic jelly without affecting the molar 
conductivity. 


THE PROPERTIES OF SOAP CuRDS 


These white opaque systems form a complete contrast to soap 
jellies. Under the ultramicroscope it can be seen that the forma- 
tion of a soap curd is due to the separation from solution of micro- 
scopic or ultramicroscopic fibers, which may be of very great 
length and which have been shown to consist of neutral hydrated 
sodium soaps. Hydrated sodium soaps in their solid stable 
form consist of these curd fibers, whereas hydrated potassium 





Tia. 4.—0.05 n Sodium behenate. Showing freshly formed curd fibers. (Mag- 
nification 600 diameters.) 


soaps form true lamellar crystals, the fibrous form being evanes- 
cent and unstable. A few curd and crystalline forms are shown 
in Figs. 3 to 7. The curd fibers possess a very high tem- 
perature coefficient of solubility, so that a commercial soap 
usually consists of such fibers enmeshing a soap sol or gel of the 
more soluble soaps. Formation of curd from a solution progres- 


424 COLLOIDAL BEHAVIOR 


sively lowers the conductivity to a value corresponding to the 
solubility at that particular temperature. The osmotic activity 
is similarly diminished, and the results are confirmed by the 
analysis of the expressed liquid. 

According to recent X-ray work of Piper and Grindley’ the 
curd fibers of sodium palmitate are built up from molecules of 
soap laid parallel to each other and placed end to end so that 
sodium comes next to sodium and paraffin chain is end on to 
paraffin chain. There are thus sheets of double sodium atoms 
44A (4.4uu) apart; that is, just twice the length of the single 
molecule as determined by Langmuir in his monomolecular films. 
Curd fibers are probably to be regarded as true crystals. It 
is important to note that anisotropic soap solutions give no 
such X-ray pictures; these are confined to crystals and curds. 

The chief theoretical result is the sharp distinction between true 
soap jellies, anisotropic liquids, and the soap curds with which 
they have been commonly confused. Gelatinization isnota proc- 
ess of crystallization in the sense of removal of colloidal particles 
from the sphere of action of the solvent, whereas the formation 
of curd or coagulum involves this separation. Presumably, jellies 
in general should be distinguished from coagula. 


THE CONDUCTION OF ELECTRICITY IN COLLOIDAL SYSTEMS 


Movement of charged carriers is identical with electrolytic 
conductivity. All carriers, whether ions, particles, or surfaces, 
must, therefore, impart conductance to the system. Theionic 
micelle was postulated and discovered on the ground of its high 
conductivity. This is only a particularly striking case of an 
“electrical double layer”? where the conductivity of both parts 
of the double layer has been at last taken into account and 
it clearly indicates that the electrical conductivity of ions and 
surfaces is unimpaired in an electrical double layer. This at 
once brings the whole problem of electrophoresis into the field 
of electrolytic migration. 

What is involved in every case is the relative movement 
between solvent and carrier, whether this relative movement be 


7Proc. Physical Soc., 35 (1923), 268. 


THE STUDY OF SOAP SOLUTIONS 425 


envisaged as electrophoresis, electro-osmosis, or ionic migration. 
Miss Laing has carefully investigated sodium oleate sols, gels, and 
curds. She finds that the “electrolytic migration” in soap solu- 
tions consists of strictly identical processes with ‘‘electro-osmo- ' 
sis” of water through a transparent soap jelly and with the 





Fia. 5.—1.0 n Potassium elaidate. Between crossed nicols showing fibers and 
true crystals. (Magnification 120 diameters.) 


“electrophoresis”? of pieces of soap jelly suspended in a soap 
solution. In each case the relative movement of, for instance, 
sodium and water is the same. 
A general formula 
* cymifi 
; b 
expresses the actual movement relative to the solvent of each 
constituent of any electrically conducting system, whether 
homogeneous or heterogeneous, electrolytic, colloidal, or involv- 
ing electrical double layers or diaphragms. The movement of 


the solvent relative to any conducting constituent is mals kilos of 


solvent per faraday of current and is termed ‘‘electro-osmosis.”’ 
n; is the number of chemical equivalents transported per faraday 


426 COLLOIDAL BEHAVIOR 


of current; c, is the concentration of that constituent in chemical 
equivalents per kilo of solvent; m; is the number of chemical 
equivalents which carry one electrical charge, whether positive 
or negative; f: is the electrical conductivity per electrical charge, 
so that mf; is the effective mobility; and yu is the total conduc- 
tivity of the solution and is equal to the sum of the concentrations 
of each conducting constituent multiplied by its mobility. From 
this standpoint, the only distinction between colloid and crystal- 
loid is the number of chemical equivalents which are associated 
with one electrical charge. The effective mobility mf; appears 
to be of the order of magnitude of an ordinary ion, although jf; 
is usually very minute. 





Fia. 6.—2.25 n Potassium laurate. Fic. 7.—3.0 N Potassium laurate. 
Between crossed  nicols, showing Between crossed nicols, showing 
“liquid crystals’’ in isotropic medium. “liquid crystals’? on cooling after 
(Magnification 100 diameters.) melting. (Magnification 100 dia- 

meters.) 


It was pointed out by Miller® in 1907 that the electrophoretic 
mobility of a colloid expressed in centimeters per volt per centi- 
meter per second is almost as great as that of a slow-moving 
organic ion, which is exactly what is found here for undissociated 
colloidal soap, in spite of its negligible conductivity. 

Using the new formula and recognizing that undissociated soap 
moves one-third as fast as the ionic micelle, it was possible 
quantitatively to predict all the values for curds even when the 


8 “ Allgemeine Chemie der Kolloide,”’ 


THE STUDY OF SOAP SOLUTIONS 427 


transport exceeded five equivalents of oleate per faraday of 
current. 

Obviously, migration experiments, whether by the Hittorf or 
direct method, do not measure the conductivity or true mobility 
of a colloidal particle but only its effective mobility or electro- 
phoresis mif:. Direct support for these views has been obtained 
from a study of the conductivity of potassium chloride in capil- 
lary cells made of quartz. 

This has led to a new conception of the electrical double 
layer. The mobile ions involved in all electrokinetic phe- 
nomena are very sparsely distributed and cover less than one 
tenth of a per cent of the surface. The contact potentials 
hitherto deduced are shown to possess but a fictitious and 
ambiguous significance, and the assumption made in all pre- 
vious mathematical treatment of this subject appear to require 
radical revision. 

In conclusion, it is seen that an adequate elucidation of the 
manifold and definite properties of soap solutions will have a 
direct bearing upon a surprisingly large number of theoretical 
considerations. 7 


REFERENCES 


1. McBatn, James W. and Taytor, Mituicent: Zur Kenntnis der Kon- 
stitution von Seifenlésungen Lésungen von “ Natriumpalmitaten,”’ 
Z. physik. Chem., 76 (1911), 179. 

2. McBarn, James W. and Taytor, Minuicentr: Uber die elektrische 
Leitfahigkeit von Seifenlosungen, Ber., 43 (1909), 321. 

3. CornisH, E. C. V.: Dichtigkeitsmessungen der Seifenlésungen, Z. 
physik. Chem., 76 (1911). 

4. Bowpen, R. C.: Studies of the constitution of soap in solution: the 
electrical conductivity of sodium stearate solutions, Trans. Chem. 
Soc., 99 (1911), 191. 

5. McBain, JAmMres W., CornisH, E. C. V. and Bowpmrn, R. C.: Studies 
of the constitution of soap in solution: sodium myristate and sodium 
laurate, Trans. Chem. Soc., 101 (1912), 2042. 

6. McBarn, J. W.: “The Mobility of Highly Charged Micelles,” Trans. 
Faraday Soc., 9, (1913), 99; Kolloid-Zeitschr., 12, (1913), 256. 

7. Bunsury, H. M. and Martin, H. E.: Studies of the constitution of soap 
solutions: the electrical conductivity of potassium salts of fatty acids, 
Trans. Chem. Soc., 105 (1914), 417. 

8. McBatn, J. W. and Martin, H. E.: Studies in the constitution of soap 
solutions: the alkalinity and degree of hydrolysis of soap solutions, 
Trans, Chem. Soc., 105 (1914), 957. 


428 COLLOIDAL BEHAVIOR 


9. 


10. 


Lt; 


12. 


13. 


14, 


15. 


16. 


We: 


18. 


19. 


20. 


21. 


26. 


Laine, M. E.: The state of potassium oleate, and of oleic acid in solution 
in dry alcohol, Trans. Chem. Soc., 112 (1918), 435. 

McBain, Jamus W. and Boxam, T. R.: The hydrolysis of soap solutions, 
measured by the rate of catalysis of nitrosotriacetonamine, Trans. 
Chem. Soc., 113 (1918), 825. 

Brepiz, F. C. and Bouam, T. R.: The hydrolytic alkalinity of pure 
commercial soaps, J. Soc. Chem. Ind., 40 (1921), 277. 

McBain, James W.: Colloidal electrolytes: soap solutions as a type, 
J. Soc. Chem. Ind., 87 (1918), 2497. 

McBatn, James W., Larne, M. E. and Trrtey, A. F.: Colloidal electro- 
lytes: soap solution as a type, Trans. Chem. Soc., 115 (1919), 1279. 
McBain, James W. and Satmon, C. §.: Colloidal electrolytes. Soap 
solutions and their constitution, J. Amer. Chem. Soc., 42 (1920), 

426. 

McBain, James W. and Satmon, C. S.: Colloidal electrolytes. Soap 
solutions and their constitution, Proc. Roy. Soc., A, 97 (1920), 44. 
McBatn, James W.: Colloids and colloidal electrolytes, Nature (March 
10, 1921), 46. 
McBatn, James W.: A novel magneto effect, Nature (March 17, 1921), 

74. 

Laine, M. E. and McBatn, James W.: The investigation of sodium 
oleate solutions in the three physical states of curd, gel, and sol, 
Trans. Chem. Soc., 117 (1920), 1506. 

McBain, James W. and Taytor, Minuicent: The degree of hydration 
of the particles which form the structural basis of soap curd, deter- 
mined in experiments on sorption and salting out, Trans. Chem. Soc., 
115 (1919), 1300. 

McBatn, James W. and Martin, H. E.: The hydration of the fibers 
eof soap curd. Part I. The degree of hydration determined in 
experiments on sorption and salting out, Trans. Chem. Soc., 119 
(1921), 1369. 

McBain, James W. and Satmon, C. S.: The hydration of the fibers 
of soap curd. Part II. The dew-point method, Trans. Chem. Soc., 
119 (1921), 1374. 


. Laine, M. E.: The hydration of the fibers of soap curd. Part III. 


Sorption of sodium palmitate, Trans. Chem. Soc., 119 (1921), 1669. 


. Kina, A. M.: The effect of high concentration of salt upon the viscosity 


of a soap solution, J. Soc. Chem. Ind., 41 (1922), 1477. 


. McBain, James W., Taytor, M. and Larne, M. E.: Studies of the 


constitution of soap solutions, solutions of sodium palmitate, and 
the effect of excess of palmitic acid or sodium hydroxide, Trans. 
Chem. Soc., 121 (1922), 621. 


. Saumon, C. S.: Direct experimental determination of the concentration 


of potassium and sodium ions in soap solutions and gels, Trans. 
Chem. Soc., 117 (1920), 530. 

Satmon, C. S.: Note on the effect of electrolytes on the constitution 
of soap solutions, as deduced from electromotive force, Trans. Chem. 
Soc., 121 (1922), 711. 


27. 


28. 


29. 


30. 


dl. 


32. 


33. 


34. 


35. 


36. 


37. 


38. 


THE STUDY OF SOAP SOLUTIONS 429 


Darke, W. F., McBatn, J. W. and Satmon, C. 8.: The ultramicroscopic 
structure of soaps, Proc. Roy. Soc., A, 98 (1921), 395. 

McBain, J. W.: Colloid chemistry of soap. Part I. Solutions. Third 
Report on Colloid Chemisiry, British Association for the Advancement 
of Science (1920), pp. 1-381. Reprinted in serial form in the Oil and 
Trade Colour Journal (1921), and the Sezfensieder Zeitung (1921). 

McBain, J. W. and Watts, E.: The colloid chemistry of soap. Part 
II. The soap boiling processes, Fourth Report on Colloid Chemistry, 
British Association for the Advancement of Science (1922), pp. 244-263. 

Fuecker, O. J. and Taytor, M.: Studies of the constitution of soap 
solutions. Sodium behenate and sodium nonoate, T’rans. Chem. Soc., 
121 (1922), 1101. 

McBatrn, J. W. and Burnett, A. J.: The effect of an electrolyte on 
solutions of pure soap. Phase rule equilibria in the systems: sodium 
laurate-sodium chloride-water, Trans. Chem. Soc., 121 (1922), 1320. 

Norris, M. H. and McBain, J. W.: A study of the rate of saponifica- 
tion of oils and fats by aqueous alkali, under various conditions, 
Trans. Chem. Soc., 121 (1922), 1362. 

McBatn, J. W. and Jenkins, W. J.: The ultrafiltration of soap solutions. 
Sodium oleate and potassium laurate, Trans. Chem. Soc., 121 (1922), 
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McBain, J. W., Harporne, R. S. and Kine, A. Miniicent: A method 
of determining the detergent action of soaps, Chem. Ind. 42 (1923) 3731. 

McBain and Darke: Trans. Faraday Soc., 16 (1920-21), 150. 

McBain, J. W.: A general conception of neutral colloids, and its 
bearing upon the structure of jellies. Trans. Faraday Soc., Nov. 
5, 1923. 

Laine, M. E.: A general formulation of movement in an electric field. 
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McBain, J. W.: The conception and properties of the electrical 
double layer, and its relation to ionic migration. J. Phys. Chem., 
28 (1924). 


CHAPTER XVII 
VISCOUS AND PLASTIC FLOW IN COLLOID SYSTEMS 


By 
EUGENE C. BIncHAm 


When Graham called the viscometer a colloidoscope, he was 
epigrammatically calling attention to the unique importance 
of flow in determining the properties of colloids. In our older 
science, properties like melting point, boiling point, solubility, 
and elasticity have played a réle the importance of which can 
scarcely be overrated. Our science has very largely developed 
out of the possibility of isolating substances in which these 
properties are sharply defined. The absence of these control 
properties in the newer colloid chemistry has constituted a 
distinct handicap. The search for such control properties holds 
out, as a result of its successful conclusion, the promise of rapid 
development in the newer science. 


APPARENT VISCOSITY 


Whereas the character of the flow of colloids was very early 
recognized as important, it must be admitted that as a control 
property viscosity has also failed to meet our proper expectations. 
Various investigators have called attention to the fact that the 
viscosity of many colloids is not constant but highly dependent 
upon the rate of shear. Elaborate investigations have served 
to prove that this difficulty does not appear when dealing with 
homogeneous liquids or gases. It is, therefore, peculiar to col- 
loids, although it may not be characteristic of all colloids. Since 
viscosities are of little value unless exactly definable, we come at 
once to the conclusion that, when one is dealing with the flow of 
colloids, it is of first importance to determine whether the vescosity 
7s independent of the shearing stress. When the so-called “vis- 
cosity”’ varies with the shearing stress, it becomes merely an 

430 


VISCOUS AND PLASTIC FLOW IN COLLOID SYSTEMS 431 


apparent viscosity, which is not a constant of the material but a 
variable with very wide limits. 

A clue to this anomaly in the flow of colloids was found by 
a study of the shear obtained with shearing stresses of different 
magnitude. For the understanding of this, we will now proceed 
to a consideration of the different types of deformation under 
shearing stress. 


TYPES OF FLOW 


Let us consider a parallelopiped of material ABCD confined 
between the imaginary planes AB and CD, as shown in section 
in Fig. 1, and acted upon by a 
shearing stress on either of the 
two planes of F dynes per 
square centimeter. Further- 
more, let us suppose that the 
distance between the planes is 
represented by rv. Several 
things may happen. 

The points in the plane AB 
may tmmediately move over in 
the direction of the shearing stress by a distance AA’, each layer 
of the material taking its part in this deformation in direct 
proportion to its distance from the plane CD. It is characteristic 
of this type of deformation that it quickly reaches its final value 
and does not increase further unless the shearing stress is in- 
creased. Onthe other hand, if the shearing stress is removed, 
the material springs back to its initial position. The elastic 
deformation AA’ = s follows the law first enunciated by 
Hooke 





s = Fer (1) 


where e is the coefficient of elasticity. In the limiting case, 
where the elasticity is zero, there would be no deformation and 
the material would be said to be perfectly rigid. It is to be 
particularly noted that elastic deformation does not depend upon 
the time factor. 

In another case, after the first elastic deformation, it is found 
that the deformation increases steadily with a constant velocity 


432 COLLOIDAL BEHAVIOR 


v, which is maintained as long as the shearing stress is applied. 
This viscous deformation follows the law 


v = For (2) 


where ¢ is the coefficient of fluidity. This law applies to all 
fluids, or perhaps it would be more nearly correct to state that 
substances are to be regarded as fluids which obey this law. 
Viscous deformation increases with the time. Glass and pitch 
are to be regarded as viscous liquids, since they obey the above 
law, whereas paint and butter are to be regarded as soft solids, 
because they do not obey it. According to this conception, the 
properties of fluidity and elasticity are quite independent, and 
fluids as well as solids may show an instantaneous elasticity of 
shape. 

When substances do not undergo continuous deformation at 
a given shearing stress, they may yet do so at a slightly higher 
stress. If a certain shearing stress f, called the “yield value,” 
is required to start the flow, and thereafter the deformation is 
proportional to the excess of shearing stress applied, we have 


v= (F — f)ur (3) 


where » is called the mobility of the material. This has been 
proposed as the fundamental law of plastic flow. As in the case 
of viscous flow, plastic flow is dependent upon the time. 

For the sake of completeness, the theoretical case should here 
be referred to in which no continuous deformation takes place 
before the ultimate strength of the material is exceeded. But 
since, in rupture, the material does not flow, it is not necessary 
to comment upon it further in a discussion of flow. 

In the study of flow, a complication arises in that these differ- 
ent types of deformation may take place simultaneously and lead 
to confusion. For example, it is well known that no substance 
obeys Hooke’s law perfectly, even far below the yield point. 
There is some deformation taking place which is increasing with 
the time, even when the shearing stress is very small. It must 
be ascribed to viscous flow. When this deformation is appreci- 
able, we have the indefinitely defined “elastic limit.’? Above 
the yield point plastic flow begins, but it is doubtless associated 
in some way with the viscous deformation which manifested itself 


VISCOUS AND PLASTIC FLOW IN COLLOID SYSTEMS 483 


at the lower stresses. If this mixed flow manifests itself in an 
apparently homogeneous substance like zinc, it is not surprising 
that it becomes quite evident in a heterogeneous substance like 
paint. In fact, it is because the solid and liquid phases of a 
mixture tend to flow through capillary spaces at different rates 
when subjected to shearing stress, that the use of the filter press 
is feasible in separating, let us say, oil from the oil cake. For 
want of a better term, this viscous flow of the dispersion medium 
through and ahead of the disperse phase has been referred to as 
seepage. 

One of the results of this mixed flow is that when the rate of 
flow is plotted against the shearing stress, curves are obtained 
which are not linear throughout their entire length, ABC in 
Fig. 2; they rather bend off toward the origin, as BD. As would 
be expected, the seepage is most noticeable in the neighborhood of 
the yield value. Were the flow of plastic materials below the 
yield value entirely viscous, the curve BD would be linear as 
demanded by equation (2), so the curve proclaims the tact which 
is proved also by experiment that the entire material is deformed 
in this kind of flow.! As the 
shearing stress is increased, the 
plastic flow becomes more and 
more important in comparison 
with the slow seepage. In the 
cases thus far investigated, the 
flow-stress curves become more 
and more nearly linear, as re- 
quired in true plastic flow, B 
equation (3). The yield value D 
is then found by extrapolating’ 
the curve AB to the point C Fra. 2. 
in the figure. 

Whereas the above view has seemed to be the simplest and 
most obvious one to take, the writer has no desire to be dogmatic, 
and refers the inquiring reader to the critical study of the subject 
by E. Buckingham.* Along with the complication due to seep- 


1 Cf. discussion of pseudo-plastic flow later. 
2 Proc. Am. Soc. Testing Materials, 21 (1921), 1154. 


434 COLLOIDAL BEHAVIOR 


age, we should most certainly refer to sl¢ppage.* Since this 
has not even yet been measured in any well-authenticated case, 
it may be passed over with a mere reference‘ for the reader and a 
caution to the experimenter. 

If one constituent of the plastic material resists continuous 
deformation at the same time that another constitutent is con- 
tinually giving way, the shearing stress exerted on the first portion 
must steadily increase and, therefore, the eiastic deformation in 
this constituent will also increase with the time elapsed; and if, 
after some time, the shearing stress is removed, a most peculiar 
effect is to be observed. The elastic deformation should imme- 
diately disappear, as stated above, but, due to the seepage, it is 
clearly impossible for all of it to disappear until seepage has 
taken place again in the opposite direction. ‘The result is that 
there is an immediate partial recovery followed by a slow recovery 
known as the elastic after-effect of Weber. ‘The strains accumu- 
lated in a solid as a result of various attempts to shear the material 
may greatly affect the deformation produced by a given stress, 
as is thoroughly well known to metallurgists. 

We are then able to distinguish the following independent 
properties and their reciprocals, which appear to be sufficient 
to characterize the different types of flow: 


e = elasticity, the reciprocal of n = rigidity 
gy = fluidity, the reciprocal of » = viscosity 
uw = mobility, the reciprocal of € = consistency 


The term ‘‘consistency”’ has heretofore been used loosely 
and there is as yet no general agreement to give it the above 
exact meaning. But the tendency of an advancing civilization 
is to give to words more precise meanings, and so to say, for 
example, that a substance has the consistency of butter is offen- 
sive so long as consistency cannot be quantitatively defined, and 
it appears quite certain that the properties of butter which 
determine its flow depend in an extraordinary degree upon the 


3 Hydraulic flow and flow through orifices should also be referred to, 
although information on these types, mostly with liquids, leads to the belief 
that no new property of matter is involved. 

4Bineuam, E. C.: “Fluidity and Plasticity,’’ McGraw-Hill Book Co., 
1922, pp. 20 et seg. . 


VISCOUS AND PLASTIC FLOW IN COLLOID SYSTEMS 4385 


temperature. The use of an exact definition should in no way 
lessen the real value of the loose use of the word, and the exact 
use of the term will always be understood whenever a numerical 
value is used. There is a body of opinion indicating that 
consistency is a measure of resistance to rapid deformation, and 
this is the sense in which the term is here employed. ‘There 
is a considerable advantage in using a term which is already 
familiar. 


THE SIGNIFICANCE OF PLASTICITY MEASUREMENTS 


Now that the anomaly referred to at the beginning of this 
chapter has been explained through the clear differentiation 
between viscous and plastic deformation, it seems proper to 
consider some of the possible ways in which a knowledge of the 
yield value and mobility may supply useful information. 

The Melting or Softening Temperature in Colloids.—When 
a colloidal substance, such as asphalt or butter, is heated it 
becomes soft, and efforts without number have been made to 
determine the melting points of such materials, and, by follow- 
ing a carefully specified procedure, it is sometimes possible to 
assign to a given substance a number which can be closely 
duplicated. But the value obtained depends in a high degree 
upon the exact procedure followed and, upon practical as well as 
theoretical grounds, it may be doubted whether these substances 
have a definite melting point. Presumably with this in mind, 
other names have been suggested, such as the “‘softening tem- 
perature.”’ Regardless of the name that may be given it, which 
is of relative unimportance, it is highly desirable that the con- 
stant involved should be definitely determinable. To fulfil this 
requirement it is not sufficient that the numerical constant can 
be duplicated by following a closely specified procedure with a 
given apparatus. To have the widest usefulness the value of 
the ‘‘constant’”’ should not be dependent in any way upon the 
dimensions of the apparatus or of the procedure used. 

Take the simple case of two substances A and B, which are 
partially soluble, the one in the other, and form a eutectic, as 
represented in Fig. 3. A given mixture having the concentration 
C will be entirely in solution at an elevated temperature, but on 
cooling to D it will reach saturation and under favorable circum- 


436 COLLOIDAL BEHAVIOR 


stances the solid phase of B will make its appearance and, 
simultaneously with the appearance of the new phase, the mate- 
rial will display a yield value under suitable conditions. In other 
words, the transformation from fluid to solid takes place at the 
point D, and this temperature is the analogue of the melting point. 
As the cooling is continued, the amount of the solid phase B 
increases, but since the crystals may be separated from each other 


A B 
Fig. 3. 


by the remaining liquid phase until the point E is reached, we 
may be prepared to find the material to be a “soft solid,” if 
subjected to test on the plastometer. At the eutectic tempera- 
ture # the liquid phase disappears and a new type of solid phase 
appears. If there is plastic deformation possible, it must be based 
on the deformation of one or both of the solid phases, whereas 
before, the deformation was largely, if not exclusively, in the 
liquid phase. The yield value and mobility temperature curves, 
therefore, both show a discontinuity at the eutectic temperature. 

From this point of view, one would expect the plasticity data 
to give information similar to that obtained from the cooling 
curves, but not in agreement with other conventional means for 
determining the so-called melting point, some of which may be 
described briefly. : 

The method recommended for butter by Dr. Wiley consists 
in floating thin discs of butter between layers of water and 
alcohol and then raising the temperature until the butter draws 
‘itself up into spheres. 


VISCOUS AND PLASTIC FLOW IN COLLOID SYSTEMS 437 


In the ball-and-ring test of the melting point (A. S. T. M. D36- 
21) a steel ball is used to force out the material which is supported 
in a brass ring. Raising the temperature at the exact rate of 
5°C. per minute, the melting point is taken as the temperature 
at which the material is extruded from the ring 2.54 cm. 

In the Saybolt melting point apparatus a series of small 
wooden sticks are held in place by means of the plastic material 
in well holes drilled in a brass plate which can be immersed in 
a vessel containing water. The melting point is taken as the 
temperature at which the sticks are released and rise to the 
surface of the water, the bath being raised in temperature at a 
rate of from 0.5 to 1.0°F. per minute. 

Still another method by Pohl consists in dipping a thermometer 
into the molten material, withdrawing it, and allowing the mate- 
rial to solidify on the bulb of the thermometer, after which the tem- 
perature is again raised at the rate of 1°F. per minute until the 
first drop leaves the thermometer. 

In these last methods it is evident that plastic flow is taking 
place. In one case the shearing stress is derived from the inter- 
facial tension, in another from the weight of the ball, inanother 
from the buoyant force acting upon the sticks, and in the last 
from the weight of the material itself. Since the shearing stress 
is very different in the different methods, and since, therefore, 
the flow must take place while the yield value has a finite value, 
it seems possible that the temperatures obtained may be quite 
anomalous, depending as they do upon the yield value and the 
mobility, as well as upon the time of heating. We may even 
imagine that, given two substances A and B, with one instrument 
the substance A will appear to be the softer of the two, whereas 
with another instrument the substance B will appear to be softer 
than A. The reason for this behavior is made clear in Fig. 4 
where the rate of deformation for the substance A is greater than 
that of B at the shearing stress F’, whereas the reverse is true at 
the shearing stress F'’’. 

As an example of this type of substance, butter may serve 
as an example. The fluidity temperature curve is linear from 
35 to 50°. But at 35.7°C. there is a discontinuity. Below that 
temperature there is found a yield value which increases in a 
linear manner as the temperatureislowered. At35.7° the fluidity 


438 COLLOIDAL BEHAVIOR 


is approximately 2.5 and the fluidity of the undercooled liquid 
butter at 32° is 2.0, but the mobility falls off very rapidly and 
reaches a value of only 0.0005 at 32°. At 17.3° the mobility 
seems to reach a zero value, and we have found that at 10° we 
were unable to obtain true plastic flow with the pressure avail- 
able. There was, however, some oil squeezed out by seepage 
which would not solidify 
at 10°. 

There is one fundamental 
difference between the cooling 
curve method and_ the 
plasticity method. The suc- 
cess of the former depends 
upon the assumption that 
equilibrium at each tempera- 
ture is secured during the 
time required for the cooling 
to take place. With the 
plasticity method no such 
assumption is necessary, for as many determinations of the 
plasticity may be made as are desired. With butter, for example, 
it has been found that, even with seeding and thorough stirring, 
the time required for reaching equilibrium may run into hours. 

On solidification, new molecules must diffuse up to the face of 
the crystal as rapidly as the liquid bathing the crystal is impover- 
ished, and this rate of diffusion is directly proportional to the 
fluidity of the medium. The difficulty still remains if the tem- 
perature of experiment is reached by raising the temperature 
instead of by a process of cooling, for, as the solid phase dissolves, 
it is necessary for the dissolved molecules to diffuse into the 
solution in order that the process of solution may continue. If 
the temperature could be raised so rapidly that diffusion did not 
result, we may even conceive of a mixture remaining plastic up 
to the temperature of melting of the highest melting component. 

The “Solubility” of Colloids.—Having shown that the plasti- 
city method may give sharp control points for colloids which 
are at least the analogues of the eutectic temperature and melting 
point, or better, saturation point of true solutions, we will proceed 
to the consideration of plasticity and the so-called solubility of 





Firq. 4. 


VISCOUS AND PLASTIC FLOW IN COLLOID SYSTEMS 4389 


colloids. When it is said that nitrocellulose is more soluble in 
acetone, let us say, than in butyl acetate, the statement does not 
mean what such a statement does in classical chemistry. In the 
first place, the solution is not a true solution at allin either solvent, 
so far as is definitely known and, therefore, these substances do 
not form a saturated solution, which can be measured. If, with 
this idea in mind, we abandon the term solubility and say that 
acetone peptizes a certain nitrocellulose, whereas water alone 
does not, we may have gained something in precision, but our 
knowledge is very unsatisfactory, since it cannot be expressed 
quantitatively. When it is stated that nitrocellulose is more 
soluble in acetone than in butyl acetate, it appears to be based in 
some way upon the fact that for a solution of the same concen- 
tration the solution in acetone flows the more readily. Since, 
however, acetone has a considerably higher fluidity than butyl 
acetate this is what we would expect, and we find ourselves in 
the possession of nothing more than a possible clue to the problem. 

Since all solutions of nitrocellulose appear to be plastic, the 
the terms “‘viscosity’’ and “‘fluidity’’ must be avoided. We come 
then, to a study of the yield value and mobility of these solutions. 
Hyden, Ross, and the author have studied solutions of Newburgh 
nitrocellulose in acetone and a 60 per cent diethyl ether and 
40 per cent ethyl alcohol mixture by weight. For a 5 per cent 
solution of nitrocellulose at 20°C. the following values of the 
yield value and mobility were obtained as compared with the 
fluidity of the pure solvent: 








Solvent Fluidity Yield value Mobility 
PUMeraiCONOlss pai. ke ew 250.00 11.50 0.86 
OW AE SS 310.00 8.00 1.99 
ae i a a shy fol as 1.24 1.43 2e31 


If the solubility is merely a matter of the fluidities of the pure 
solvents, then we should expect to find the mobilities in the 
two solvents to have the same ratio as the fluidities of the solvents 
themselves. From the above table it is evident that the mobility 
in acetone is far greater than we should expect. This and 


440 COLLOIDAL BEHAVIOR 


other facts not yet referred to indicate that in the process of 
peptization a portion, although perhaps only a very small portion, 
of the solute goes into true solution. According to this hypoth- 
esis, acetone is regarded as a good solvent, not simply because 
of its high fluidity, but because it actually dissolves a greater 
portion of the nitrocellulose into true solution. (In the use of 
the term “true solution” we must avoid too hastily assuming 
that the colloid particles are completely broken up into simple 
unassociated molecules.) 

When a substance dissolves in some solvent without dissociation 
or combination, the fluidity concentration curve is nearly linear.® 
When, however, the solute unites with the solvent, the fluidity 
concentration curve generally departs from the linear and the 
amount of the sagging of the curve may be used to calculate® the 
composition of the solvate as well as the amount of combination 
which has taken place. The mobility concentration curve of 
nitrocellulose is an extreme example of this type, indicating 
that the nitrocellulose molecule is highly solvated. But the 
mobility does not give us a measure of the solubility. 

On the other hand, the yield value most certainly arises from 
the portion of the material which is not in true solution. As 
the solution is heated, the yield value grows smaller and finally 
disappears, at 43°C. in the case of acetone, and 71°C. for ether- 
alcohol extrapolated. At these temperatures we must assume 
that Newburgh nitrocellulose goes entirely into true solution. It 
has also been found that the rate of decrease of the yield value with 
the temperature is the same in both solvents, which seems to indi- 
cate that the temperature coefficient of solubility is the same in 
both solvents and at a given temperature the amount undissolved 
is measured by the yield value. Finally, it may be remarked 
that the temperature of zero yield value is not a melting point, 
although it may be regarded as analogous to the melting tem- 
perature. It may be generally true that the lower the tempera- 
ture of zero yield value the greater the solubility, but since the 
solubility ratio may vary with the temperature, the yield values 
at a given temperature should alone be compared. 


5 BrncHaAM, HE. C.: Lib. cit., pp. 160 e¢ seq. 
6 Jdem., pp..176 et seq. 


VISCOUS AND PLASTIC FLOW IN COLLOID SYSTEMS 441 


POLAR AND NON-POLAR COLLOIDS 


Having thus briefly considered colloids of the emulsion type, 
it is well to take up colloids of the suspension type, since they 
differ from the former in important respects. Friedlinder’ 
supposed that small percentages of suspended substances do not 
affect the viscosity. This confusing result is a consequence of 


Ream se 0) ef) i |e 
Nee ee 


2.6 
2.4 





fae [is 
meee Se 

Sid alae 
SE eee 
MEO eo a 
ee eee 
Std 
(coe Ne 


OS eee 
MeN || Nb | 


oo 10 20-50-40 5060 1080 90 100° 
Weight Percentage Lithopone 





ne 
1.0 


60 


Mobility 
ield Value 







Fic. 5.—The variation in the yield value and mobility obtained by dilution of a 
suspension of lithopone in linseed oil with more linseed oil at 25°C. 


the hyperbolic character of the viscosity-concentration curve. 
As soon as we plot fluidities or, still better, mobilities, we find 
that the curve is linear, as shown in Fig. 5 for lithopone suspended 
in linseed oil, so that very dilute suspensions do not present any 
anomaly whatever. This type of curve has been confirmed 
using substances of the most diverse type. 

So far as is now known, the mobility-concentration curve 
for non-polar substances does not depart from linearity, even at 
high concentrations of the disperse phase. This is in marked 


7™Z. phystk. Chem., 24 (1897), 152. 


442 COLLOIDAL BEHAVIOR 


contrast to the behavior of polar colloids, and this may serve as 
a distinguishing test between them. The mobility curve reaches 
a zero value at a finite concentration which seems to be intimately 
related to the pore space in the disperse phase, for, on purely 
theoretical grounds, we should expect to reach zero mobility with 
close packing without interlocking. According to this view the 
mobility-concentration curve is completely defined by the fluidity 
of the medium and the pore space of the disperse phase; and thus 
in the concentration of zero mobility we have another control 
point of some possible value. For spheres of equal size, cubical 
close packing would correspond to a concentration of 52.4 per 
cent by volume. We cannot suppose that lithopone is made up 
of spheres of equal size, nevertheless it is not without interest 
to observe that the concentration of zero mobility is 79.6 per cent 
by weight, which is 45.6 per cent by volume. 

At the higher concentrations (above 48 per cent in lithopone 
and linseed oil, as shown in the figure) the yield value increases 
in a linear manner with the concentration. Assuming that this 
behavior is general for non-polar colloids, the concentration of 
zero yield value as found by extrapolation is significant as well 
as the manner in which the yield value increases with the concen- 
tration. The concentration of zero yield value is dependent 
upon the particle size of the disperse phase, but also to an 
extraordinary degree upon the adhesion between the medium and 
the disperse phase. Thus, a paint made up from a neutral 
mineral oil and lithopone was found to have its yield value 
lowered 80 per cent by the addition of only 0.2 per cent of 
oleic acid. 

Since it has been customary to measure what we have here 
called ‘‘apparent fluidities’’ instead of mobilities, it may be 
remarked in passing that the apparent fluidities are always lower 
than the mobility and, with a very long capillary and a small 
pressure head, the apparent fluidity-concentration curve may 
approach the curve FA in Fig. 5 and may, therefore, be used to 
find the concentration of zero yield value. 

As noted in the figure, there is evidence that the yield-value- 
concentration curve is not linear throughout its entire length. 
Very dilute suspensions show a very small though real yield 
value, as indicated by the part of the curve BEO. For greater 


VISCOUS AND PLASTIC FLOW IN COLLOID SYSTEMS 443 


clarity of expression, the flow in suspensions having concentra- 
tions up to the point A may be referred to as pseudo-plastic. 
In explanation it may be said that the causes in operation in 
pseudo-plastic flow are probably different from those in true 
plastic flow. In the latter it is assumed that the particles touch 
each other, while in the former this is certainly not necessarily 
the case. In pseudo-plastic flow the energy is dissipated by the 
distortion of the stream lines of flow around the particles. In 
plastic flow this flowing of the medium ahead of the particles 
suspended is regarded rather as a correction term arising from 
seepage, but much more work is needed on this whole matter. 
From what has preceded it would be inferred that all colloidal 
materials should show a yield value, but this conclusion is hardly 
justified. Gardner and Holdt® have shown that varnishes have 
a very small or even negligible yield value. Adding to a varnish 
several per cent of stearic acid or talc, as is done in making dental 
impression waxes, does not introduce a noticeable yield value, 
although it does greatly decrease the fluidity. Though at first 
surprising, this is quite in accordance with what was observed 
in the case of lithopone and linseed oil. The fluidity of these 
impression compounds is a linear function of the temperature 
over a range of 15 or more degrees, the fluidity reaching a zero 
value at a definite temperature. This temperature of zero 
fluidity is again a temperature of transition between viscous 
and plastic flow and it may serve as a useful control temperature 
in the case of very viscous liquids. A study of the plasticity 
constants below the transition temperature would also probably 
give useful information. As in the other transition temperatures 
which have been described in this chapter, this is certainly not a 
true melting point, because there is no sudden increase in fluidity 
at the transition point. Elsewhere the writer has attempted 
to give an explanation of this transition point.?® 
- Perhaps even this brief treatment will suffice to suggest 
to the reader ways in which plasticity data can be utilized to 
advantage in colloid chemistry. The author must disclaim any 
attempt to give an exhaustive summary of those properties 
which might be studied with profit from the viewpoint of plastic- 


8 Paint Manufacturers’ Assoc. of the U. S., oe No. 127. 
9 J. Phys. Chem., 28 (1924), 263. 


444 COLLOIDAL BEHAVIOR 


ity. Hardness, malleability, ductility, and tensile strength 
tests all involve plastic flow. Important properties in the arts, 
scarcely recognized among standard physical properties, are 
length and shortness, which are applied to the most diverse 
materials, such as iron, dough, glue, paint, etc. These properties 
gain an entirely rational explanation on the basis of fluidity 
and plasticity relations. Apparently, quite distinct from the 
hardness which we recognize in metals is that “hardness’’ in 
certain fossil resins which makes them so highly valued in the 
manufacture of varnish. It can scarcely be doubted that this 
hardness is in some way connected with the resistance to flow 
exhibited in the varnish film and perhaps also in the original 
varnish resin. 


AUTHOR INDEX 


(References to Vol. I are in lightface type; references to Vol. II in boldface type. 


A 
Adam, 168, 169, 210 
Adams, 221 
Adkins, 261 


Aitchison, 489 

Alexander, 360, 382, 386, 485, 498, 
800, 804 

Allen, 262 

Alsberg, 581-604 

Anderson, 381, 481 

_v. Antropoff, 260 

Archer, 495, 497, 498, 501, 759 

Arkwright, 92 

Armstrong, 265, 273, 362-376 

Arpin, 600 

Ashley, 480 

Atchley, 119, 120, 122 

Atsuki, 644 

Auerbach, 403 

Aultman, 682 


Bach, 369 

Bachman, 580 

Bachmann, 381, 383, 386, 390, 400 

Bacon, 544 

Baer, 522, 523 

Bahr, 521 

Bailey, 581, 588, 592, 597 

Baker, 512, 621, 623, 625, 787, 788 

Balderston, 720 

Ballard, 586, 588, 591 

Bancroft, 70, 82, 215, 221, 223, 231, 
258-275, 278, 296, 359, 360, 
378, 381, 388, 392, 401, 449, 
490, 493, 498, 518, 530, 532, 
550, 776 


Bang, 338 

Banks, 763 

Barker, 186, 210, 515 

Barratt, 384, 386 

Barrowcliff, 655 

Barsch, 512 

Bartholomew, 693 

Bary, 672 

Bastin, 458 

Bates, 538 

Batley, 517 

Bauman, 505 

Bayliss, 237, 363, 374, 419 

Bean, 502 

Beans, 19, 77, 359 

Beaver, 359 

Bechhold, 324, 347, 356 

Bechold, 227, 406, 407 

Becker, 712 

Beckerath, 247 

Becquerel, 772 

Beebe, 282, 283, 285, 290 

Beer, 246 

Behr, 575 

Beilby, 484, 494, 498, 518 

Bellows, 341 

Bengough, 494 

Bennett, 133 

Benson, 231 

Benton, 258, 261, 265, 266, 281, 282, 
283, 287 

Berglund, 699 

Berl, 270, 280 

Berliner, 267 

Bermann, 313 

Bernick, 268 

Bernstein, 674 

Berthelot, 259 


Bertrand, 369 


ll AUTHOR INDEX 


Bevan, 640 

Bhatnagar, 631 

Biederman, 391 

Bigelow, 606 

Billaz, 492 

Billitzer, 93, 326. 327, 347, 349 

Biltz, 325, 419 

Bingham, E. C., 430-444, 537, 595 

Bingham, K. E., 497 

Bishop, 139, 681 

Black, 537 

Blatnager, 221 

Bleeker, 492 

Bleininger, 567 

Blish, 581, 592 

Blizard, 521 

Blockey, 720 

Bloor, 687 

Blum, 779 

Bobilioff, 652 

Bodenstein, 270, 279 

Bodlander, 270 

Boeke, 446 

Bogue, 22, 23, 41, 384, 385, 390, 
575, 732-757, 767 

Boiry, 682 

Bolton, 759 

Boltzmann, 379 

Bone, 513, 520 

Borcherdt, 569 

Bordar, 551 

Borjeson, 389, 394 

Borrowman, 653 

Bosworth, 340, 786, 787, 797 

Boudguard, 512 

Boynton, 490 

Bradford, 384, 386, 403, 408, 457 
756 

Bradley, 828 

Bragg, 218, 221 

Brassert, 521, 522 

Bray, 260 

Brearley, 489 

Bredig, 92, 260, 263, 268 

Bremer, 586 

Breur, 521, 522, 529 

Briggs, 133, 223, 224, 227, 228, 580 


Broadbridge, 517 

Broche, 521 

Brossa, 298, 315, 317, 347, 349 
Brown, 126, 156, 210, 211 
Browne, 728 

Buchner, 391, 401 
Buckingham, 433 

Buel, 594 

Bugarszky, 25 

Bullock, 772 

Binz, 513 

Burdick, 258 

Burgess, 492, 513 

Burns, 264, 265, 267, 281, 283 
Burrell, 532 

Burton, 70, 72, 75, 76, 78, 123-141 
Bury, 517 

Bitschli, 380, 400 

Buxton, 85, 324 


C 


Cain, 486 

Calvin, 589, 590, 592 

Cameron, 227, 230, 646 

Campbell, 261, 612 

Capitaine, 573 

Capstaff, 772 

Carnaval, 464 

Carpenter, 788 

Caspari, 674, 675, 679 

Chaney, 523 

Chapin, 408 

Charpy, 520 

Cheng, 210 

Chick, 598, 796 

Child, 214, 220 

Clark, F. W., 462 

Clark, G. L., 153, 155, 188, 193, 197, 
201, 210, 216, 220, 225 

Clark, J. D., 459, 465 

Clark, W. M., 513, 787 

Clarkson, 699 

Clayton, 215, 221, 226 

Clowes, 195, 211, 226 

Cobb, 563 

Cohn, 33, 36, 97, 587, 588, 590, 595, 
788, 789, 794 


Coker, 489 
Coleman, 517 
Connstein, 367 
Coombs, 211 
Copeland, 808 
Cornu, 456 
Cottin, 600 
Cottrell, 530, 553, 557 
Coulter, 92 
Courtauld, 641 
Creuss, 612, 620 
Crosby, 221 
Cross, 640 
Cullen, 75 
Currie, 140 


D 


Dacey, 544 
Dachnowski, 504, 532 
Daguerre, 758 

Dahle, 802 

Dakin, 33, 799 
Damm, 513 

Daniels, 490, 578 
D’Arsonval, 551 
Davidheiser, 578 


AUTHOR INDEX lil 


Dewar, 264 

Diesz, 396 

Dittmer, 526 

Ditz, 512 

Déehle, 394 

Doherty, 589, 590 

Doisy, 122 

Dolid, 654 

Dollfus, 572 

Donath, 512, 520 

Donnan, 1, 41, 122, 186, 210, 2138, 
220, 223, 419, 715 

Dougherty, 281, 282 

Downey, 800 

Downs, 527 

Dox, 603 

Draper, 201, 202, 218, 220 

Dreaper, 465 

Drucker, 187, 210 

Dubosc, 643 

Duclaux, 644, 646 

Dunstan, 493 

Dupré, 147, 205, 210 


EK 


Eastlack, 19, 77 


Davidsohn, 37, 338, 339, 340, 344 Eaton, 122, 656, 681 
Davies, 155, 198, 197, 201, 210, 216, Eccles, 262 


220, 225 Eddy, H. C., 221 
Davis, C. E., 767 Eddy, W. G., 221 
Davis, J. A., 545 Eder, 782 
Davis, J. D., 504-533, 517, 757 Edser, 568 
Davis, R. O. E., 481 Edwards, 486 
Davy, 3438 Eggert, 773 
Dede, 227 Eggerth, 341 


DeHaas, 608, 611 
DeKruif, 82, 92, 340 
Demanski, 406 
Denham, 650 

Dennis, 720 
Dennstedt, 513 
Denton, 610, 611 
Desch, 484, 502 
Devaux, 164, 165, 210 
DeVries, 656, 676, 681 
Dew, 282, 283, 289 


Eggink, 677, 678 
Ehrenberg, 480, 505 
Kinstein, 61, 62, 126, 205, 211 
Eitle, 770 

Elissafof, 133 

Elliott, 350, 382, 768, 777 
Ellis, 78 

Engelder, 270 
Englehardt, 532 

Engler, 369 

Eschholz, 556 


iv AUTHOR INDEX 


Esselen, 627-651 
Evans, 522, 681 
Ewing, 194, 210, 211 


F 


Fajans, 246, 247, 248, 253 

Faraday, 267, 346 

Feldman, 170, 210 

v. Fellenberg, 608, 609, 618 

Fenn, 588, 590, 595 

v. Fenyvessy, 600, 601 

Field, 509 

Field, 382, 753 

Fieldner, 504-533, 578 

Fink, 270, 280 

Finkel, 201, 202, 218, 220 

Fischer, E., 25, 371 

Fischer, M., 242, 422 

Fischer, R., 513, 514, 519, 520, 521 
522, 528, 529 

Flade, 383 

Flecker, 298, 347 

Flemming, 574 

Flusin, 662, 664, 666 

Foerster, 394, 566 

Fol, 674 

Folin, 686 

Foreman, 585 

Fraas, 402 

Fraenkel, 500 

Frankenberger, 253 

Frazer, 260, 510 

Fremont, 490 

Frémy, 606, 607, 608 

Freundlich, 70, 72, 73, 75, 76, 84, 
93, 94, 95,: 96, 210, 235, 
241, 247, 249, 286, 287, 297- 
323, 327, 405, 669 

Friedel, 584, 599 

Friedemann, 84 

Frieden, 343 

Friedlander, 441 

Friedmann, 347 

Friend, 494 

Fry, 481 


Galecki, 337 

Galeotti, 603 

Gallun, 757 

Gansser, 100, 122 

Gardner, 443 

Gaudechon, 259 

Gauger, 2738, 274, 281, 288, 288, 290 

Gaunt, 674 

Gayler, 499 

Gedroiz, 480 

Geer, 681 

Gernsdorff, 581, 584 

Giampalmo, 603 

Gibbs, 182, 205, 210, 220, 233 

Gibson, 643 

Gile, 481 

Gilfillan, 265 

Gillett, 482-503 

Giolitti, 485, 489, 490 

Girardin, 591 

Gladstone, 262 

Glaser, 529 

Glover, 641 

Gluud, 513, 514, 519, 526, 528, 529 

Godschot, 520 

Goldthwaite, 611, 612, 619, 620 

Goldtrap, 599 

Gomolka, 278 

Gore, 606 

Gortner, 350, 351, 383, 394, 585, 
588, 589, 590, 592, 595, 596 

Gosney, 367 

Gottwald, 571 

Gouy, 253 

Gowland, 491 

Grafton, 193 

Graham, 24, 324, 342, 346, 399, 400, 
515, 572, 784 

Green, 682 

Greider, 682 

Griffin, 216 

Griffith, 489 

Grimaux, 395 

Grimm, 631 

Grindley, 424 


AUTHOR INDEX 


Groh, 272 

Groh, 354, 584, 599 
Groschuff, 572 
Guess, 593 

Gully, 505 

Gurber, 122 
Guthrie, 593 
Gutman, 122 
Gyorgy, 241 


H 


Haber, 238, 247 

Hadley, 514 

Haigh, 489 

Hall, 509 

Haller, 237 

Hamburger, 98, 122 

Hamer, 674 

Hammarsten, 784, 786 

Hamor, 544 

Hansen, 491 

Harbeck, 267 

Harder, 460 

Hardy, 25, 35, 37, 77, 149, 150, 210, 
380, 381, 383, 541, 587, 588, 
589, 593, 789, 790 

Harger, 513 

Harkins, 38, 142-211, 213, 216, 220, 
225, 383, 541 

Harned, 270 

Harries, 678, 679 

Harris, 419, 594, 715 

Harrison, 386, 387, 403, 628 

Harting, 391 

Hatfield, 497 

Hatschek, 210, 379, 407, 449, 450, 
464, 672 

Haughton, 497 

Hausding, 506 

Hawk, 800 

Hayes, 462 

Hedges, 132 

Heidelberger, 105, 122 

Heisig, 808 

Helmholtz, 140, 250, 253 

Henderson, 100, 117, 588, 590, 595 


Hendry, 33, 36, 97, 789, 794 

Hennis, 570 

Henri, 141, 324, 653 

Henry, 270 

Herbert, 606 

Herschell, 761 

Herter, 800 

Heubner, 353 

Heuser, 629, 631, 639, 651 

v. Hevesey, 77 

Heyd, 522 

Heynemann, 278 

Hibbard, 490 

Hibbert, 630 

Higson, 781 

Hildebrand, 201, 202, 212-221 

Hilditch, 265, 373 

Hilgard, 480 

Hill, 516, 630 

Hillyer, 214, 220, 223 

Hinrichsen, 682 

Hirst, 630 

Hitchcock, 31, 33, 34, 57, 274 

Hober, 122 

Hochstetter, 264 

Hodgson, 775 

Hoffmann, 261, 512 

Holdt, 4438 

Holmes, 214, 220, 222-232, 395, 
404, 408, 465, 580, 646 

Holn, 802 

Hooker, 224 

Hopkins, 369, 370 

Horne, 553 

Horovitz, 246 

Horton, 368 

Houghton, 666 

Howard, 606 

Howe, H. M., 480 

Howe, W. F., 490, 494 

Howell, 383 

Hoyt, 484, 490 

Huff, 553 

Hiifner, 100, 122 

Hulett, 262, 417, 516 

Hull, 218, 221 

Hummel, 462 


vi AUTHOR INDEX 


Hummelbaur, 445 
Humphrey, 211, 496 
Hunt, 761 

Hunter, 339 
Hutchinson, 517 


Ikeda, 268 

Illingsworth, 513, 519, 520 
Ingalls, 491 

Inglis, 489 

Ipatieff, 270 

Ipatiew, 263 

Tredale, 272, 292, 355, 356 
Irvine, 464, 630 


Jacobs, 353 

Jaeger, 209, 211 

Jago, 582 

James, 491 

Jane, 680 

Jarisch, 342 

Jeffries, 486, 495, 497, 498, 501 

Jessen-Hansen, 591, 595 

Johannsen, 520 

Johns, 604 

Johnson, F., 495, 496 

Johnson, J. B., 490 

Johnson, L., 329-337 

Johnson, W., 491 

Johnstin, 610, 611 

Johnston, 461 

Jones, 281, 292, 513, 578, 581, 584, 
604 

Jordis, 382, 570, 572 

Jorgensen, 598 


K 


Kahlenberg, 667 
Kanter, 572 
Kaplan, 520 
Kappen, 270 


Kastle, 271, 367 

Katorski, 337 

Katz, 378, 515 

Kauffman, 404 

Keeny, 492 

Keith, 203, 204 

Kelber, 273 

Kennet, 759 

Kenrick, 231 

Keppeler, 506 

Kern, 723 

Kindscher, 682 

King, 188, 190 

Kinney, 517, 523, 545 

Kirchh of, 660, 662, 664, 666, 669, 
672, 674, 679 

Kleeman, 567 

Knapp, 716, 720, 727, 728, 760 

Knerr, 490 

Knuth, 296 

Kohl, 567, 568 

Kohler, 456 

Kohlrausch, 566 

Kohlschiitter, 249 

Kommers, 489 

Konigsberger, 464 

Konno, 499 

Kossel, 287 

Kramer, 122, 266 

Kratz, 660, 661 

Kroger, 566, 682 

Kruyt, 134, 242, 259, 320, 677, 678 

Kugelmass, 646 _ 

Kunitz, 18, 41, 42, 46, 49, 51, 52, 59, 
60, 64 

Kuster, 280 

Kutscher, 338 


L 


Labrouste, 168, 210 
Lachmann, 262 
Lachs, 241 

Laing, 385, 386, 421 
Lamb, 124, 260 
Lamplough, 516 
Landenberger, 602 


AUTHOR INDEX Vil 


Landis, 275 

Langeberg, 490 

Langenvin, 126 

Langmuir, 38, 97, 149, 150-152, 166, 
158,7 189,°193, 210, 213, 216, 
220, 225, 260, 278, 280, 287, 
283, 541 

Lantsberry, 502 

Laqueur, 789 

Lea, F. C., 489 

Lea, M. C., 759, 769 

LeBlanc, 682 

LeChatelier, 388 

LeGray, 759 

Leick, 402, 403, 405 

Lenher, 573 

LePrince Ringuet, 515 

Lewes, 520, 526 

Lewis, 72, 222, 261, 523, 799 

Liebermann, 25, 264 

v. Liebig, 573 

Liesegang, 407, 446, 774 

Lilienfeld, 650 

Linder, 35, 82, 246, 248, 324, 325 

Lindgren 445-465 

Lloyd, D. J., 382 

Lloyd, F. E., 652 

Lodge, 553, 557 

Loeb, J., 6, 7, 18, 22, 23-69, 76, 78, 
BOale va vo, 119, 122, 136, 
342, 349, 356, 357, 358, 398, 
586, 589, 590, 617, 709, 713, 
745, 749, 756, 757, 767, 779, 
790, 792, 793, 795, 812 

Loeb, R. F., 59, 122 

Loening, 84 

Loevenhart, 271, 367 

Lorenz, 769, 770 

Lottermoser, 95, 247, 324, 325, 327, 
761 

Lucas, 600 

Liters, 582, 584, 587, 588, 589, 593, 
596, 599, 601 

Lumiére, 782, 783 

Lundal, 662, 663 

Lunge, 267, 527 

Lunn, 182, 210 


Ltippo-Cramer, 342, 759, 762, 763, 
765, 767, 771, 772, 774, 775, 
776, 777, 778, 780, 781 


Lyon, 492 
M 
McAdam, 489 
McAdams, 523 
McBain, 136, 388, 384, 385, 386, 


410-429, 540, 756 
McCall, 643 
McCance, 490 
McCollum, 578, 683-699 
MacDonald, 699 
McGavack, 575, 578 
McGee, 549, 561 
Mack, 516 
McLean, 98, 105, 108, 112, 122 
MeNair, 612, 620 
MacNider, 603 
Maddox, 759 
Madsen, 797 
Maffia, 249 
Mahler, 515 
Malcomson, 569 
Malengreau, 338 
Maltezos, 141 
Manabe, 33 
Manchot, 498 
Mangin, 606, 607 
Manson, 570 
Mare, 445 
Marcelin, 210 
Mardles, 650 
Mark, 496 
Marshall, 122 
Martin, 598, 796 
Marzetti, 681 
Mathews, 489, 490, 757 
Matthews, 221 
Matula, 33, 248 
Maury, 221 
Maxted, 268, 269, 281 
Maynard, 799 
Mees, 771, 774, 778 
Mellor, 455 


vill 


Mendel, 699 

Menz, 350 

Merica, 492, 499 

Meserve, 532 

Meunier, 383, 724, 729, 730 

Meyer, G., 775, 776 

Meyer, H., 502 

Michaelis, 23, 37, 90, 93, 101, 122, 
233-257, 320, 338, 339, 340, 
344, 582, 584, 587, 631, 787, 
790 

Middleton, 481 

Miller, 576, 577 

Milligan, 296 

Millikan, 547 

Milner, 188, 210 

Minor, 630, 635 

Mitchell, 690 

Moeller, 384 

Mond, 273, 278 

Moore, B., 685 

Moore, C. J., 481 

Moore, H. F., 489, 532 

Moore, W. E., 220, 226, 296 

Morgan, 167, 370 

Morse, 602 

Miller, 426 

Mulliken, 194 

Mylius, 570, 572 © 


N. 


v. Nageli, 380, 383 
Nasmith, 582, 587 
Nathansohn, 327 
Neill, 105, 122 
Neilson, 271 
Neisser, 84 

Nell, 406 

Nelson, 274 
Nesbit, 461, 549 
Neuberg, 391 
Neuhausen, 101, 122, 400 
Neumann, 595, 600 
Neunier, 221 
Newburg, 699 
Newman, 221 





AUTHOR INDEX 


Newmann, 198 
Nicholas, 404 
DeNiepce, 758 
Nietz, 774 
Niggli, 446 
Niklas, 445 
Niwa, 557 
Noddack, 773 
Nolf, 358 
Norris, 420 
North, 682 
Northrop, 70-97, 340, 787 
Norton, 568 


O 


Oakes, 757, 767 

Ober, 96, 246, 248 

Oberfell, 532 

Odell, 531 

Okuda, 804 

O’Leary, 359 

O’Neill, 544 

Osborne, 581, 582, 585, 594, 599, 
601, 603 

Ostromislenski, 680 

Ostwald, 214, 220, 225, 240, 378, 
408, 422, 505, 506, 535, 588, 
589, 595, 596, 601, 644, 670, 
671, 680 

Ovitz, 515 


P 


Palmer, 119, 120, 122, 266, 273, 802 

Paneth, 246, 248 

Park, 699 

Parmelee, 568 

Parr, 510, 514, 520 

Parsons, 221, 274 

Pascal, 558 

Patrick, 400, 575, 578 

Pauli, 23, 25, 33, 248, 298, 347, 360, 
661, 780, 792, 796 

Peabody, 539 

Pearson, 513 

Pease, 261, 269, 283, 284, 285 


AUTHOR INDEX 1x 


Pechstein, 787, 790 

Peek, 551, 562 

Pelizzola, 655, 681 

Pellat, 250 

Pemsel, 263 

Pepe iio, 1G, 77, 131, 133, 250, 
415 

Perrott, 517, 521, 522, 523, 545 

Peterson, 588 

Philpot, 490 

Phragmen, 502 

Pickering, 215, 220, 225 

Picton, 35, 82, 246, 248, 324, 325 

Pierson, 133 

Pincussohn, 248 

Piper, 424 

Place, 545 

Plateau, 214, 220 

Pockels, 165, 210 

Pohle, 437, 654, 665, 679 

Polanye, 496 

Polanyi, 237 

Pollard, 267, 273, 289 

Polvogt, 689 

Popp, 712 

Porges, 87 

Porritt, 670, 674, 682 

Porter, 132, 515 

Posnjak, 401, 528, 660, 661, 669 

Potts, 220, 223 

Powell, 510, 524, 525 

Powis, 78, 80 

Priestly, 489 

Procwer 10, Lip 12, 17,.22, 41, 58, 
98, 122, 378, 398, 590, 700-731, 
779 

Punter, 643 


Q 
Quincke, 223, 380, 484 

R 
Rahn, 803 


Ralston, 516, 517 
Ramsay, 273, 278 


Rankine, 403 

Raoult, 666 

Rassow, 394 

Rathbun, 553, 559 

Ray, 580 

Rayleigh, 140, 165, 205, 210, 211, 220 

Reichardt, 607 

Reichert, 563, 601 

Reid, 266 

Reiger, 403 

Reinders, 770 

Reiner, 227 

Reinke, 402 

Reitstotter, 315, 316, 353 

Renwick, 772, 773 

Richards, 211, 218, 221 

Richardson, 296 

Richter, 296, 515 

Rideal, 263, 272, 295, 359 

Ripperton, 626 

Roark, 603 

Roberts, 210 

Robertson, 28, 25, 228, 788, 789, 
791, 792, 793, 796 

Robinson, J. G., 544 

Robinson, W. O., 481 

Rocosolano, 272 

Rodewald, 401 

Roeber, 491 

Rogers, 447, 450 

Rohland, 505, 568 

Rohm, 713 

Rolfe, 490 

Rona, 240, 241, 243, 247, 248, 320, 
582, 584, 587, 631 

Rosenhain, 490, 494, 497, 499 

Rothe, 247 

Rowley, 531 

Ruder, 492 


Sackur, 789, 792 

St. John, 521 
Saklatawalla, 486, 494, 497 
Samec, 360 

Sand, 493 


X AUTHOR INDEX 


Sandqvist, 420 Simon, 407, 464 
Sang, 492 Singh, 613 
Sargent, 262, 513 Sinkinson, 513 
Sato, 593, 604 Skerrett, 544 
Schaum, 761 Skraup, 34 
Scheed, 496 Slade, 760 
Scheibler, 607 Smalley, 487 
Schener, 500 Smith, 405, 590, 753, 756 
Scheringa, 631 v. Smoluchowski, 93, 94, 126, 131 
Scherrer, 386, 497, 756 Snow, 626 
Schlaepfer, 220, 226, 532 Snyder, 586 
Schloesing, 480 Soddy, 266 
Schénbein, 268 Séldner, 784, 786 
Schorger, 632 Somers, 455 
Schreiber, 522, 523 Sommer, 801 
v. Schréeder, 401 Sérensen, 23, 38, 97, 598, 745 
Schultz, 720, 721 Spence, 660, 661, 679 
Schulze, 35 Spencer, 643 
Schumann, 761 Spilker, 526, 527 
Schurecht, 567 Spring, 325 
Schuster, 405 Sproxton, 643 
Schtitze, 797 Squibb, 260 
Schulze, 238 Starling, 122 
Schwalbe, 632, 635, 651 Stearn, 633 
Schwerin, 568 Stephens, 678 
Scott, A., 445, 449, 454 Stericker, 563-580 
Scott, G. S., 545 Stern, 318, 681 
Scott, H., 499 Sterry, 771 
Seidell, 623 Stevens, 658, 679, 681 
Selvig, 509, 520 Stewart, 370 
Seguin, 343 Stieglitz, 152 
Senter, 269 Stock, 278, 279 
Seyewetz, 782, 783 Stéckigt, 639 
Seymour-Jones, 343 Stockings, 513 
Sharp, 585, 588, 589, 590, 592, 595, Stokes, 541 

596 Stopes, 514 
Shaw, 494 Stoughton, 490 
Sheppard, 220, 350, 534-545, 748, Strong, 546, 562 

753, 758-783 Stuart, 494 
Sherman, 521 Stubel, 383 
Sherwood, 597 Sucharipa, 611, 624 
Shibley, 88 Sullivan, 456 
Shields, 273, 278 Sulnam, 568 
Shinkle, 532 Sutcliff, 22 
Shory, 568 Sutermeister, 627 
Simmonds, 689 Svedberg, 126, 415, 773 


Simmons, 681 _ Sveshnikoff, 502 


AUTHOR INDEX X1 


Sweet, 768, 777 
Swinden, 505 
Szyszkowski, 187, 188, 189, 210 


T 


Talbot, 759, 782 

Tammann, 271 

Tarr, 605-626, 756 

Jaylor, H. §., 261, 263, 264, 265, 
267, 269, 273, 275, 276-296 

Taylor, W. W., 70, 90, 96, 97 

Teague, 324 

Thiessen, 504-533 

Thole, 493 

Thomas, 195, 324-361 

Thompson, 381, 525 

Thomson, J. J., 233 

Thomson, W., 196 

Thorpe, 262, 494 

Thum, 485 

Tideswell, 513 

Tiebackx, 342 

Tingle, 631 

Tisdall, 122 

Todokoro, 593, 604 

Tollens, 606, 608, 611 

Tolman, 459, 465, 633 

Tomkins, 662, 664, 666, 667, 669 

Touplam, 551 

Traube, 187, 210 

Tribe, 262 

Trivelli, 760, 762, 763, 770, 772, 776 

Trood, 492 

Tucan, 450 

Tuttle, 655 

Twiss, 680, 682 


Ueno, 269 

af Ugglas, 339 
Upson, 589, 590, 592 
Upton, 488 

Urban, 575 

~ Uschkoff, 771 


Vail, 571 

Van Arsdale, 296 

Van Bemmelen, 380, 399, 480, 505, 
576 

Van Dam, 803 

Van der Waals, 379 

Van Duin, 242, 259 

Van Heurn, 658, 663 

Van Rossem, 654, 673 

Van Slyke, D. D., 98-122, 340, 368 

Van Slyke, L. L., 786, 787, 788 

v. Vegesack, 419 

Vernaci, 485 

Vignon, 514 

Vinson, 697 


WwW 


Walcott, 555 
Walker, 523, 553, 557 
Walpole, 385, 406 
Waltenberg, 499 
Warburg, 98, 109, 113, 122, 554 
Warren, 550 
Washburn, 401 
Weatherwax, 603 
Weaver, 599 
Weber 653 
Wegelin, 360 
Weigert, 773 
Weimarn, 384, 387, 391, 392, 393, 
395, 449 
Weinwurm, 601 
Weiser, 377-409 
Weiss, 527, 761 
Weistgerber, 526 
Westgren, A., 495 
Westgren, N., 502 
Whatmough, 187, 210 
Wheeler, 513, 514, 541 
Wheldale, 369 
Wherry, 447, 450 
Whitby, 652-682 
White, 504, 506, 532 
Whitney, M., 466-481 


xl AUTHOR INDEX 


Whitney, W. R., 96, 246, 248 Wolski, 505 

Whymper, 588 Wood, 35, 584, 587, 588, 589, 592, 
Wiegand, 682 593, 712 

Wiegner, 480 Woodhouse, 650 

Wightman, 763, 766, 772, 776 Woodman, 584, 585 

Wiley, 436 Worden, 651 

Williams, 480, 638, 805 Wright, 568 

Williamson, 461 Wu, 98, 105, 108, 112, 122 

Willows, 210 

Wilson, C. T. R., 548 Y 


Wilson, J. A., 1-22, 23, 41, 58, 59, 
76, 98, 122, 590, 709, 713, 


723, 728, 749, 754, 767, 779, \2mada, 617 
808-829 Yamakami, 789 
Wilson, L. P., 637, 641 Yancey, 610 
Wilson, R. E., 221, 223, 231 
Wilson, W. H., 12, 58 eras 
Winchell, 464 
Windisch, 313 Zeisberg, 274 
Winmill, 515, 516 Zeigler, 406 
Winn, 655, 678 Zocher, 405 
Wirth, 531 Zoller, 784-807 
Wislicenus, 628 Zsigmondy, 90, 240, 350, 351, 379, 
Witte, 556 380, 383, 386, 388, 579, 798 
Wolff, 401 | Zucker, 122 


Wollman, 644 Zunz, 353 


SUBJECT INDEX 


(References to Vol. I are in lightface type; references to Vol. II in boldface type.) 


A 


Activated sludge, 808-829 
Adsorbate, 234, 277 
Adsorbent, 234 
Adsorption, 178-194, 233-257, 258- 
275, 493 
and catalysis, 282-286 
and catalytic activity, 293-295 
and coagulation, 248-250 
and pressure, 288-293 
general equation, 579 
in liquid systems, 295-296 
ionic discharge and coagulation, 
250-255 
of electrolytes, 242-248 
of silica gels, 576-579 
ammonia, 578 
nitric oxide, 578 
petroleum, 577-578 
sulfur dioxide, 578 
water, 576-577 
Albumin, 298-317 
Alkali-blue, 307 
Allotropic silver, 769 
Alloys, 482-503 
Amorphous metal hypothesis, 494- 
498 
Anthraxylon, 507, 508, 509 
Apparent viscosity, 430—431 
Arsenious sulfide, 326, 337 
Avogadro’s number, 74 


B 


Bag houses, 548 

Baking, 597-598 

Barley, 601-602 

Bichromated colloides, 782-783 
Bitumens, 519 


Xlli 


Blood, 98-122 

Body fluids, 98-122 
carbon dioxide tension, 114—118 
electrolyte distribution, 118-122 
electrolytes, 99-101 
proteins, 99-101 
solution laws, 101-114 

Bread, 598 

Brownian movement, 72, 73, 

126-131, 546 
Butter, 436, 803 


C 


Casein, 31-57, 339, 784-807 
addition compounds, 799 
coagulation, 794-798 
combining capacity, 791 
decomposition products, 799 
hydrolysis, 794 
isoelectric, 790 
preparation, 786—789 
protective action, 798 
pure, 788 
salts, 789-794 

Catalase, 364 

Catalysis, 258-275 
autotoxic, 271 
contact, 276-296 
enzyme, 362-376 

Cataphoresis, 326 

Cellulose, 627-651 
acetate, 647-650 
adsorption, 630 
chemical characteristics, 631 
classification, 627 
composition, 629-630 
constitution, 629-630 
distribution, 627 
electrical properties, 630-631 


123, 


X1V 


Cellulose, ethers, 650-651 
formation, 628-629 
formula, 630 
hydration, 632 
hydrolysis, 634-635 
liquefication, 628-629 
mercerization, 6833-684 
nitrate, 642-647 
nitro, 644-647 
oxidation, 638-639 
structure, 628-629 
Centrifuging, 548-549 
Cereals, 581-604 
Cheese, 803-804 
Chromatic emulsions, 230 
Chromic oxide gel, 377 
Classification of solutions, 416 
Clay, 461 
casting, 566-567 
flotation, 568-569 
purification, 567-568 
Coagulation, 94-95, 136-139 
Coal, 504-533 
constitution, 506-512 
colloidal nature, 511-512 
composition, 507-511 
gas, 532 
origin, 504-506 
peat, 504-506 
physico-chemical behavior, 512- 
518 
adsorption of gases, 515-516 
extraction, 512-515 
moisture, 516 
Trent process for cleaning, 517— 
518 . 
washing by froth flotation 516— 
517 
sulfur in, 524-526 
suspensions, 538-541 
Coalescence, 491-492 _ 
Cohesive force, 88, 89, 93 
Coke, 518-526 
adsorption of gases, 521 
combustibility, 521-523 
desulfurization, 525-526 
formation and structure, 518-524 


SUBJECT INDEX 


Coke, sulfur in, 524-526 
water in, 520-521 
Colloidal electrolytes, 411-421 
fuel, 584-545 
gold, 90 
Colloidon, 81, 91, 92, 357 
Congo red sol, 305 
Consistency, 434 
Contact catalysis, 276-296 
and adsorption, 282-286 
specificity, 286-288 
Corona, 551, 554, 558 
Corrosion, 4938-494 
Cotton, 628 
Coulomb’s law, 252 
Cream, 802-803 
Critical potential, 251 
Crystalline gel, 383 
Crystallization, 483-487 


D 


Dairy industry, 784-807 
Dielectric constant, 670-671 
Dietary, 683-699 
Diffusion, 406-409 
Disperse systems, 123-141 
Brownian movement, 126-131 
molecular motions, 129-131 
osmotic considerations, 127-129 
coagulation of sols, 136-139 
Perrin’s distribution law, 131-132 
settling, 124-126 
theory of fluctuations, 131 
Distribution law, 131-132 
Donnan equilibrium, 1-10, 4446, 
53-56, 96, 98, 122, 378, 398, 
728, 729, 792 
Dopplerite, 505 
Dough, 595-597 
Dross, 487 


E 
Edestin, 31 


Egg albumin, 28-31, 36, 37, 57, 62, 
63, 82, 341, 355, 358 


SUBJECT INDEX XV 


Elastic after-effect, 434 
Elasticity, 402, 434 
of silicates, 565 
Electro-osmosis, 425 
Electrophoresis, 425 
Electroplating, 492-493 
Emulsification, 212-221 
Emulsifying agent, 214-219 
Emulsions, 197-205, 212-221, 122- 
232 
cracking, 219-220 
creaming, 229-230 
emulsifying agent, 214-219 
fogging, 766 
grainless, 762 
grain size, 763 
making and breaking, 228-229 
recognition of types, 227-228 
ripening, 762, 772 
silver halide, 760—764 
theories, 223 
types, 214-219, 225-227 
Enzymes, 362-376 
as synthetic agents 375 
intermediate complexes, 374 
lipase, 367 
oxidases, 368-374 
saccharo-clasts, 365-366 
urease, 368 
Epidermis, 700 
Euglobulin, 305, 312, 318 


¥ 


Fatigue, 489 
Ferric oxide sol, 298-308, 326-348 
Films, 169-176, 212-213 
Fixateur, 540 
Flotation, 568-569 
Flour strength, 594 
Fluidity, 434, 537 
Foams, 222-232, 534 
Formaldehyde-casein, 799 
Free energy, 143 
Froth, 230-231, 534 
Fruit jellies, 605-626 

acid in, 612-620 


Fruit jellies, pectin in, 605-612 
sugar in, 620-625 


Fuel, 534—545 


general problem, 534-536 

history and bibliography, 543-545 
processing, 542-543 

properties, 543 

special case, 536 


G 


Galolith, 799 
Gases, 546-562 
cleaning by centrifuging, 548-549 
by ionization, 550-552 
by screening, 548 
by settling, 549-550 
Gelatin, 26-92, 297, 310-382, 732- 
757 
amphoteric behavior, 749-753 
hydrolysis, 737-741 
in photography, 758-783 
isoelectric, 752-756 
manufacture, 733—737 
reactions, 749-752 
sol and gel forms, 756-757 
Gelatinous crystals, 388, 389 
precipitates, 377-409 
formation, 389-397 
structure, 377-389 
Gels, 38, 58-61, 353, 377, 389, 450 
formation, 389-397 
non-elastic, 399, 404 
properties, 400 
diffusion, 406-409 
elasticity, 402-404 
optical, 404—406 
vapor pressure, 400—402 
silica, 573-580 
adsorption by, 576-579 
drying, 575-576 
improvements, 580 
isoelectric point, 574-575 
setting time, 573-574 
structure, 579-580 
swelling, 397—400 
Glanzstoff silk, 637 


Xvi 


Gliadin, 581-585 
Globulin, 31 
Glucosides, 366 
Glue, 355, 732-757 
manufacture, 733-757 
clarification and filtration, T41— 
743 
deliming, 737 
drying, 745-748 
evaporation, 743-745 
finishing, 748-749 
hydrolysis, 737—741 
liming, 734-736 
preparation, 733—734 
raw materials, 733 
scientific control, 749-757 
soaking and washing, 734 
Gluten, 585-593 
“development,” 591-592 
effect of electrolytes, 587-591 
good, 586 
hydration, 591 
poor, 586 
theories on quality, 592-593 
Glutenin, 585 
Gold number, 350-355 
sols, 297, 338, 350, 359 


H 


Hardening agents, 381, 498-503 

Heterogenious equilibria, 1-22 

Hide photomicrographs, 701-705, 
721, 723 

Hook’s law, 432 

Humin, 505-506 

Hydration of cellulose, 632 

Hydraulic flow, 434 

Hydrogen acceptor, 369 

Hydrolysis of cellulose, 634-635 

Hydrophile sols, 297-323 


I 
Ice-cream, 804-806 


Inclusions, 487 
Inhibition number, 355, 356 


SUBJECT INDEX 


Ionic micelles, 412 
Ionization of gases, 550-562 
Tron minerals, 462 
Isoelectric gelatin, 382 
point, 25 
silica gel, 547, 548 


J 


Jellies, 377-409 
equilibria, 10-19 
formation, 389-397 
by cooling, 390 
by dialysis of sol, 395 
by metathesis, 395-397 
by precipitation from sol, 393- 
395 
of concentrated jellies, 390-393 
of fruit pectins, 614-617 
non-elastic, 399, 404 
structure, 377-389 
swelling, 397—400 


K 
Kinetics, 123-141 
L 


Latent image, 761, 768-773 
development, 773-777 
peptization, 776 

Latex, 652-657 
adsorption, 657 
coagulation, 655-657 

Leather, 700-731 

Liesegang’s rings, 407, 465 

Light-oil vapors, 532 

Limestone, 461 

Lipase, 367 

Lipoclasts, 364 

Liquid crystals, 426 


M 


Maize, 602-603 
Melting point, 435 


SUBJECT INDEX 


Membrane equilibria, 1-10, 39-41 
potentials, 41-49 
Mercerization, 633-634 
Metabolic processes, in foodstuffs, 
692-699 
in the body, 684-692 
Metacolloid, 447, 452-453 
Metals, 482—503 
adsorption, 493 
amorphous hypothesis, 494-498 
coalescence, 491-492 
corrosion, 493—494 
crystallization, 483-485 
electroplating, 492-493 
fatigue, 459 
hardening, 498-503 
interference with crystallization, 
485-487 
nuclei for crystallization, 483-485 
particle size, 486-489 
rate of crystallization, 483-485 
retention by slags and mattes, 491 
sonims, 487-491 
Milk, 785-804 
casein free, 786-799 
condensed, 800—802 
constitution, 785-786 
dried, 802 
evaporated, 800-802 
homogenized, 786 
pasteurized, 797, 800 
protein-free, 804 
remade, 802 
Millets, 604 
Minerals, 445-465 
amorphous, 448, 451-452 
colloform, 447 
colloidal behavior, 453-465 
criteria for colloid state, 448-449 
crystals from gels, 449-450 
deposition of iron, 462-463 
exhibiting colloidal behavior, 
446-453 
gels, 450 
metacolloid, 447, 452-453 
nomenclature, 450 
structure, 447 


XVil 


Minerals, tables, 450-453 
Mineral sizing, 571 
Mobility, 182-136, 434, 537 
Mutarotation, 405 
Mutual precipitation, 82-93, 324- 
345 
protective action, 346-361 
reactions, 324-361 


N 


Night-blue sol, 306, 307 
Nitrocellulose, 440, 644-647 
Non-polar colloids, 441-444 
Non-tannins, 723 

Nucleic acid, 594 


O 


Oats, 603 
Oil-mist, 534 
Optical properties, 404-406 
Ore deposits, 445-465 
Organic sulfur, 524 
Orientation theory, 148-169 
Osmotic activity of soaps, 412 

pressure, 49-58, 65 

of rubber, 667—669 

Ossein, 733 
Oxidase, 364, 368-374 
Oxycellulose, 638-639 
Oxygen acceptor, 369 


Us 


Paper making, 635-638 
parchment, 636 
size, 571 
Paraglobulin, 315, 316 
Parapectin, 607 
Particle size, 71, 72-75, 486, 487 
Pauly silk, 637 
Peat, 504-506 
Pectic acid, 609 
Pectin, 606-612 
orange, 610 
Peptization, 334, 537 
Petroleum, 577-578 
Phase rule, 182, 233, 234-235 


XVlll 


Photographic after-processes, 780- 
781 
bichromated colloids, 782-783 
development, 773-777 
emulsions, 760-764 
fixing and hardening, 777-780 
latent image, 768-773 
process, 758-783 
vehicles, 764-768 
Photohalides, 769 
Plastic flow, 430-444 
melting and softening point, 435- 
438 
significance, 435-440 
solubility, 438-440 
Plasticity of coal suspensions, 539 
Plauson colloid mill, 537 
Poisons, 266, 278 
Polar colloids, 441-444 
Pore size, 381 
Potential difference, 35, 42, 78, 80, 
93, 96 
Preservation, 696 
Promoter action, 263 
Protective action, 90, 342, 346-361, 
764-768, 798 
in rocks, 455 
Proteins, colloidal behavior, 39-69 
action of salts, 63-69 
membrane equilibria, 39-41 
membrane potentials, 41-49 
osmotic pressure, 49-58 
swelling, 58-61 
viscosity, 61-63 
Proteins, crystalloidal behavior, 23— 
39 
chemical behavior, 23-35 
solubility, 35-39 
Proteoclasts, 364 
Protopectin, 609 
Protoplasm, 694 


R 


Reductase, 364 
Refractive index, 406 
Rice, 604 


SUBJECT INDEX 


Rigidity, 434 
Ringelmann scale of density, 547 
Ripening, 697-698 
Rotation, 585 
Rubber, 652-682 
as a semipermeable membrane, 
666-667 
compounded, 682 
elastic properties, 682-633 
latex, 652-657 
osmotic pressure, 674-676 
solubility, 678 
solutions, 672 
solvent bound by, 672-673 
swelling, 657-671 
viscosity, 674 
vulcanization, 680-682 
vulcanized, 633-665 
Rye, 599-601 
proteins, 599-601 


S 


Saccharo-clasts, 364, 365-366 
Sachs-Georgi test, 318, 319 
Salts, 63-69 
Sanitation, 808-829 
Screening, 548 
Seepage, 433, 443 
Sensitization, 297-323 
Settling, 71, 72 
Sewage, 809-828 
aeration vs. putrefaction, 815-816 
degree of dispersion, 811-812 
dewatering, 810--811 
effect of aluminum sulfate, 816- 
818 
effect of pH value, 813-815 
effect of temperature, 818-821 
filtering, 812-813 
purification, 809-810 
seasonal changes, 821-822 
treatment, 822-828 
Silica, 563-580 
gels, 573-580 
adsorption, 576-579 
drying, 575-576 


SUBJECT INDEX 


Silica gels, improvements, 580 
isoelectric point, 574-575 
setting time, 573-574 
structure, 579-580 

sols, 572-573 
Silicates, 563-571 
action of salts, 569 
cataphoresis, 566 
conductivity, 566 
deflocculating effects, 566-569 
elasticity, 565 
gardens, 571 
solution, 563—564 
ultramicroscopic examination, 565 
viscosity, 564-565 
Silicic acid, 330, 332-336, 380 
Silver halides, 760-764 
adsorption, 776 


development and _ developed 
image, 773-777 
sol, 304, 309 


visible and latent images, 768-773 
Skin, 700-731 

bating, 711 

constitution, 700—703 

conversion to leather, 716-731 

depilation, 704-711 

drenching, 711-713 

grain, 701 

pickling, 713-716 

preparation, 704-716 

puering, 710 

structure, 700-703 

swelling, 708 
Slippage, 434 
Smoke precipitation, 546-562 

by centrifuging, 548-549 

by cleaning, 548 

by ionization, 550-552 

by settling, 549-550 

by unidirectional current, 552-561 
Soap, 213-220, 410-429 

and theory of colloidal electro- 

lytes, 411-421 

conduction of electricity, 424-427 

crystals, 425, 426 

curd fibers, 422, 423, 425 


Soap, forms, 410 
Jellies, 421 
sols, 411-414 
structure, 421-423 
Softening temperature, 435 
Soil colloids, 466-481 
adsorptive power, 475, 476 
behavior, 474-477 
chemical nature, 468-471 
composition, 470, 471 
conditions in soil, 477 
determination, 472-474 
mechanical analysis, 467 
moisture equivalent, 477 
number, 478, 479 
origin, 466-467 
plasticity, 474, 475 
size, 478, 479 
Sols, coagulation, 136 
silica, 572-573 
Solubility, 438-440 
Soluble ferments, 363 
Sonims, 487-491 
Spreading coefficient, 174 
Stabilization, 537 
agent, 329 
of coal suspensions, 538-541 
Starch, rye, 601 
wheat, 593-594 
Stokes’ law, 547 
Structure, 377-389 
amorphous, 387 
crystalline, 386 
fibrillar, 383 
filamentous, 384 
grainy, 383 
honeycomb, 381 
of cellulose, 628-629 _ 
of silica gels, 579-580 
of soaps, 423-424 
streptococcal, 384 
supermollecular, 383 
Sulfur sol, 328 
Surface energy, 142-211 
active bodies, 239-242 
adsorption, 181-194 
colloid systems, 196-197 


X1x 


XX SUBJECT INDEX 


Surface energy, emulsions, 197—205 
equilibria, 19-22 
films and spreading, 169-175 
heat of adsorption, 178-181 
hydrogen ion concentration, 194— 
196 
impurities, 177 
insolubility, 176-177 
monomolecular films, 148 
nonspreading, liquids, 176 
orientation theory, 148-169 
polar groups, 175 
spreading on metal, 177-178 
tension, 142, 211, 213, 233-257 
thermodynamics, 205 - 
Suspensions, 70-97 
charge, 75-81 
coagulation, 94-95 
cohesive force, 93 
particle size, 71—75 
precipitation, 82—93 
protein, 97 
valence, 95-97 
Suspensoid particles, 548 
Suspensoids, 546-562 
Swelling, 58-61, 66, 67, 397-400 
and dielectric constant, 670-671 
and protein content of rubber, 679 
nature, 669 
of rubber, 657-671 
in vapors, 665-666 
osmotic pressure, 667-669 
theory, 709 
Syphilitic serum, 318 


‘ie 


Tannin, 311, 348, 344, 345, 722 
Tanning, 716-731 
basic chrome, 720-722 
theories, 727-731 
vegetable, 722-727 
Tar, 526-533 
composition, 527-528 


Tar, cumarone resins, 529-530 
emulsions, 531-532 
mists, 530-532 
sources and characteristics, 526- 
529 
Trent process, 517-518 


U 


Urease, 364, 368 


V 


Valence, 95-97 
Vibration, 404 
Viscogen, 802 
Viscose, 639-642 
Viscosity, 61-63, 214, 434 
of fuel, 538, 540 
of silicates, 564-565 
Viscous flow, 480-444 
types, 431-435 
Visible image, 768-773 
Vulcanized rubber, 680-682 
aging, 681 
fiber, 636-637 
swelling, 663-665 
viscosity, 676-678 


W 
Wassermann test, 318, 319 
v. Weimarn theory, 395 
Wheat, 581-599 
specifi¢ rotation, 585 
Y 


Yield point, 541 
value, 432 


Zein, 602-603 




















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